General of the Army Omar N. Bradley, a World War II hero who was the last of the nation's five-star generals, died yesterday in Manhattan. He was 88 years old.

General Bradley had come to New York to attend a dinner of the local chapter of the Association of the United States Army. He was taken to St. Lukes-Roosevelt Hospital in midtown in a private car at 7:15 P.M., accompanied by his wife, Kitty, and three military aides, a hospital spokesman said. At 7:35 P.M. he was pronounced dead by Dr. Stephan Lynn, the director of the hospital's emergency services, who said the cause of death was cardiac arrest.

General Bradley became the only remaining five-star general in 1969 after the death of former President Dwight D. Eisenhower. A Pentagon spokesman said the General had remained officially on active duty until his death. as do all five-star generals . He attended President Reagan's inauguration in January in a wheelchair.

'The Honest Mechanic'

Making up in military competence what he lacked in battlefield glamour, Omar Bradley won four stars in World War II and the sobriquet of ''the honest mechanic.'' He was also known as the ''G.I.'s General'' because of his concern for the ordinary soldier.

He commanded, successively, a division, a corps, an army and finally the 12th United States Army Group in Europe, which numbered more than 1.3 million combat troops of four armies. In this capacity he was the senior commander of American ground forces in the June 1944 invasion of Europe and the subsequent defeat of the Nazi forces on the Western Front.

It was General Bradley who linked up with Marshal Ivan Stepanovich Konev of the Soviet Union on the banks of the Elbe River on April 25, 1945, a dramatic meeting that symbolized the eclipse of German arms.

Earlier, on April 6, when the Germans' defeat appeared inevitable, the general had raised the Stars and Stripes over the frowning fortress of Ehrenbreistein, across the Rhine from Coblenz, and declared that the Germans could have no doubt about the war's outcome.

''This time we shall leave the German people with no illusions about who won the war - and no legends about who lost the war,'' he said. ''They will know that the brutal Nazi creed they adopted has led them ingloriously to total defeat.''

In plowing across France from the Normandy landings of June 1944, and through Germany to the Elbe, General Bradley achieved a reputation as a brilliant tactician, the foundations of which had been laid in his campaigns in North Africa and Sicily. The essence of his tactics was that the best way to fight a modern battle was ''slow and sure.''

''Don't let this blitzkrieg business fool you,'' he once said. ''Today we can move our troops into position much faster than ever before. We can throw a whole division (13,000 soldiers) 150 miles a day, instead of 15 as in the last war. And we can exploit our victories with even greater speed.

''But the actual fighting of the battle itself is a different proposition. That's the same old ground battle fought by the soldier on foot, and it takes almost as much time as it ever did.''

Tall at just over 6 feet, erect, lanky, bespectacled and bonyfaced, General Bradley was a commander the G.I.'s liked for the care he took with their lives - and because he looked the part of an infantryman. In the field he wore an old, stained trench coat, his G.I. trousers were stuffed into paratroop boots, and his field cap was unpretentious. His voice, a Missouri drawl, was rarely raised in anger. He gave the impression of being a plain, homely, stable man, which indeed he was. Less Flamboyant Than Patton

A.J. Liebling, who covered a number of the general's campaigns, described him as ''the least dressed-up commander of an American army in the field since Zachary Taylor, who wore a straw hat.'' And contrasting him with the flamboyant Gen. George S. Patton Jr., the late Mr. Liebling wrote:

''After the Green Hornet, with his ruddy, truculent face and his beefy, leather-shathed calves, the new general, lanky and diffidently amiable, seemed a man of milk.''

At the same time, however, he impressed a war correspondent as ''a tough, knotty fighter with the tremendous sledge-hammer persistence of General Grant, the shrewdness of a New England horse trader and the personal dignity of character and integrity that can be campared only to the same spacious qualities shown always by Gen. Robert E. Lee.''

He also possessed enormous self-confidence. General Bradley recounted the following colloquy that took place when, with Maj. Gen. William B. Kean Jr., he was drawing up an officer roster for the Normandy invasion:

'' 'What a helluva responsibility this is for you and me to be pulling off the biggest invasion of the war.' ''Kean nodded and stared at the map of Europe on the wall. 'But Bill,' I said frankly, 'who in the Army knows more about it than we?' '' Lacked Combat Experience

Remarkably, General Bradley had entered the war without combat experience. A ''book general'' and the product of an Army establishment that placed a high premium on honesty and honor, he had spent his prewar years in routine professorial assignments. And by 1940 he was an obscure lieutenant colonel in civilian clothes who rode a bus to work in the old Munitions Building in Washington.

But being a ''book general,'' and the habit of composure that went with it, paid dividends in the war, for he planned each battle and had confidence in his decisions and in his men. To him war was a series of mathematical problems, and he went about the business of reaching his solutions by methods that had been tried and proved.

Born in the hamlet of Clark, Mo., on Feb. 12, 1893, Omar Nelson Bradley was the son of an underpaid schoolteacher, who died when his son was 13. The boy was named Omar for a Missouri newspaper publisher and Nelson for the family doctor. In Moberly, Mo., where the youth grew up, he acquired a love for hunting and fishing and was known for shooting a gun expertly. His high school yearbook described him as ''calculative.''

He went to West Point because his Sunday school superintendent suggested it as the best choice for a poor boy. His class at the United States Military Academy, that of 1915, has become known as the class the stars fell on. It provided more than 30 generals in World War II, including General Eisenhower and Gen. George E. Stratemeyer, Air Force commander in the China-India-Burma theater. Athlete at the Point

Cadet Bradley played football and baseball. As an outfielder he had a rifle throwing arm. He was graduated 44th in a class of 164. General Eisenhower, a classmate, graduated 25th in the class.

The serious and shy second lieutenant served a tour of duty along the Mexican border in 1916 and received a temporary promotion to major in World War I without seeing service outside the United States.

After the war, he drew duty as a teacher of military science and tactics at South Dakota State College, and in 1920 he was posted to West Point for four years as an instructor in mathematics. These followed the well-worn groove of a professional soldier in peacetime: teaching courses and taking them; duty at the Command and General Staff School and the Army War College. His most glamorous tour was a hitch in Hawaii.

In 1939, General Bradley was assigned to the General Staff in Washington. Ten years earlier his work at Fort Benning, Ga., had caught the attention of Lieut. Col. George Catlett Marshall, and in 1941 General Marshall plucked Lieut. Col. Bradley out of Washington and sent him to Fort Benning to convert the tiny Infantry School there into a huge center capable of handling 14,000 officer candidates at a time.

General Bradley did the job with commendable dispatch and without raising his voice. Later he trained the 82d and 28th Divisions for combat. In Pursuit of Rommel

In February 1943, the situation in Tunisia was deteriorating after the Anglo-American landing in Morocco and Algeria in November 1942. The combined forces had narrowly failed to take Tunis by a coup de main to catch Gen. Erwin Rommel's Afrika Korps in the rear as Gen. Bernard Law Montgomery's British army had driven it across Libya. GET FIRST ADD

The British, Americans and French were bogged down in mud, cold and discouragement in Tunisia, and there had been much criticism of the quality of the American command in operation.

General Bradley became deputy commander of the United States II Corps, fighting in the Tebessa area under General Patton. His main duty, however, was to act as General Eisenhower's eyes and ears along the entire front.

The presence of a senior officer out of the chain of command is always irksome to the responsible commanders, and this occasion was no exception. But General Eisenhower appraised his men correctly when he wrote of General Bradley: ''He was a keen judge of men and their capabilities and was absolutely fair and just in his dealings with them. Added to this, he was emotionally stable and possessed a grasp of the larger issues that clearly marked him for high office.''

Generals, junior officers and G.I.'s who were dug in among the hills of Tunisia soon became familiar with the grave, low-voiced officer who peered over his glasses in a fatherly fashion as he made it clear to them that he was not on hand to criticize, but to gather information to prevent another setback such as the Americans had suffered at Kasserine Pass.

General Bradley was placed in command of II Corps after General Patton had received another assignment, and he led it to the capture of the Nazi-occupied French naval base of Bizerte. He was promoted to lieutenant general. Proceeded to England

When General Patton's Seventh Army and the British Eighth Army, commanded by Field Marshal Montgomery, landed in Sicily, General Bradley still commanded the United States II Corps. Before the campaign was over General Marshall, the Chief of Staff, notified General Bradley that he was to proceed to England, where he was to prepare to command the First United States Army, then muster with British forces for the Normandy invasion across the English Channel.

When he could spare the time, the General was with the troops in field inspections, watching them run obstacle courses and engage in mortar practice. ''I will see you on the beaches,'' he told the G.I.'s.

Although Field Marshal Montgomery was in direct command of the assault landings in Normandy on June 6, 1944, the command of the First Army and ultimately of the 12th Army Group was held by General Bradley through the remainder of the fighting in France and Germany. He was ashore fewer than 24 hours after the first Allied units hit the Normandy beaches.

The First Army, numbering 21 divisions, began its breakout from the coastal regions late in June behind a tremendous carpet of air bombardment. On Aug. 1, the Allied ground force command changed. Field Marshal Montgomery was given command of the British 21st Army Group, while the 12th Army Group was placed under General Bradley, who was responsible only to General Eisenhower.

In the Normandy beachhead, Field Marshall Montgomery's British and Canadian force had the primary mission of holding the anchor of the line in the Caen area. Meanwhile, plans were made for the American forces to execute an end run around the German defensive positions. Patton Broke Through

General Patton's Third Army managed a breakthrough at Avranches and fanned southward and finally eastward and northward to close in on the rear of the German Seventh Army at Falaise. Although supporters of Field Marshal Montgomery have pointed out that he was facing more formidable opposition than the 12th Army Group, General Bradley felt that the British should have made a greater attempt to close the narrow Falaise gap through which the greater part of the German Seventh Army managed to squeeze to temporary safety. As it was, more than 70,000 Germans were caught in the trap.

While the British and Canadian armies were pushing northward along the English Channel, Paris was recaptured by the Second French Armored Division and other elements of the First French Army, aided by General Patton's V Corps and the French Forces of the Interior. As the Germans retreated toward the Rhine, hopes ran high for a quick end to the war in Europe.

September 1944 was, as General Bradley put it, ''the month of the Big Bust.'' Paris had fallen, and by Sept. 14 the front line extended from a region north of the English Channel port of Dunkirk through Antwerp and Aachen and Metz down to the Swiss and Italian borders.

On Sept. 3, American tanks broke across the German border near Aachen, and General Bradley pulled up stakes at Dreux, a few miles east of Paris, and established his army group command post at Verdun.

But the fast-moving Americans had outrun their supply lines and lost their momentum. For the next two months, General Bradley and the impatient General Patton were to wait at the German border for gasoline and ammunition. The dash for the Rhine had fallen just short of success. Bradley vs. Montgomery

There ensued a lengthy period of tug-of-war between General Bradley and Field Marshal Montgomery in the matter of priority for supplies. German resistance had stiffened on the British front as well as in front of General Bradley's men, and General Eisenhower was called upon to make a decision between the British proposals for operations and those proposed by General Bradley. Field Marshal Montgomery wanted to ram a spearhead through to the industrial Ruhr Valley. General Bradley favored a broad advance along the line into Germany.

The Germans, however, were massing for a desperate attempt to break through the Ardennes to capture the port of Antwerp. Field Marshal Gerd von Rundstedt had accumulated 600 tanks for this last garrison finish.

''In the face of this astonishing build up, I had greatly underestimated the enemy's offensive capabilities,'' General Bradley recalled in his memoirs. ''My embarrassment was not unique, for it was shared not only by the army commanders but by Montgomery and Eisenhower as well.''

When the German blow fell on Dec. 16, the 12th Army Group was caught without a division in reserve. Remnants of four German armies participated in the Battle of the Bulge, but while many units were overrun, Bastogne held and the Germans were too weak to exploit their initial success. General Eisenhower found it advisable to assign to the command of Field Marshal Montgomery such United States troops as were pushed north of the bulge. The troops were returned to General Bradley's command when the emergency was over.

The check administered to the 12th Army Group in the Battle of the Bulge led to pressure from the British to return Field Marshall Montgomery to his former position as commander of the ground forces. General Bradley told General Eisenhower flatly that he would not serve under Field Marshall Montgomery and that ''you must send me home, for if Montgomery goes in over me, I will have lost the confidence of my command.'' It was Winston Churchill who poured oil on the troubled waters. Crossed at Remagen

General Bradley's men crossed the Rhine at Remagen on March 7, were across in strength by March 22 and sped on toward the heart of Germany. A total of 325,000 German prisoners were captured in an encircling movement south of Essen and Dortmund. United States troops joined with the Soviet forces at Torgau on April 25, and General Bradley paid a courtesy visit to the Russian commander, Marshal Konev, who entertained the American commander with a troupe of female ballet dancers whom he identified as members of the Red Army. Later General Bradley invited Marshal Konev to his headquarters, where Jascha Heifetz entertained them with violin solos.

After Germany's capitulation, General Bradley returned to Washington and took over as head of the Veterans Administration from 1945 to 1947. He then became Chief of Staff of the Army and served two terms as chairman of the Joint Chiefs of Staff, departing in 1954. He was made a five-star general in 1950.

Besides General Eisenhower, three other men in the history of the United States have attained the title of General of the Army: Henry H. Arnold, Douglas MacArthur and George C. Marshall. John J. Pershing was awarded five stars and the title ''General of the Armies'' by an act of Congress in 1919. The only other man to have held that rank was George Washington.

After stepping down - General Bradely did not retire because Generals of the Army are considered as always available for recall to active duty - he joined the Bulova Research and Development Laboratories. He was later named board chairman of its parent company, the Bulova Watch Company. He was also on the board of the Food Fair Stores and of Metro-Goldwyn-Mayer Inc.

General Bradley celebrated his last birthday on Feb. 12 at Fort Bliss in El Paso, where he and his wife had been living since 1977. The general was recovering from a viral infection in February and had earlier been confined to a wheelchair because of knee problems going back to his days on the West Point football team.

He married Mary Elizabeth Quayle in 1916, a year after his graduation from West Point. They had a daughter, Elizabeth. Mrs. Bradley died in 1965, and the next year he married Esther Buhler, known as Kitty, who survives.