Michael O'Laughlen seems to have been in the wrong place at the wrong time. Perhaps his first mistake was growing up on the same street in Baltimore that John Wilkes Booth lived on. As he achieved adulthood (he was born in 1840), he became a manufacturer of ornamental plaster before joining, as did so many of the other conspirators, the Confederate Army, from which he was discharged in 1862. Upon returning to Baltimore, he worked as a clerk in a family feed business.
Perhaps because of their boyhood connection, John Wilkes Booth recruited O'Laughlen in the late summer of 1864 to participate in the plan to kidnap Pres. Lincoln. On Monday 13 March 1865, Booth sent O'Laughlen a telegram urging him to come to Washington. Two days later, the group of conspirators, O'Laughlen included, met at Gautier's Restaurant to consolidate their plans. Booth's first plan was to intercept Lincoln's carriage en route to a play (this plan failed when Lincoln changed his schedule); the second plan was to kidnap him while he attended a play at Ford's Theater. O'Laughlen was to extinguish the lights at the Theater, but that part was abandoned (it was more than just flipping a switch!). On Saturday 25 March, Booth sent O'Laughlen another telegram telling him to "get word to Sam" (presumably Samuel Arnold) and to be in Washington the following Wednesday morning. Whether he did or not remains unknown (at least to this author).
Maj. Kilburn Knox and two other witnesses testified at O'Laughlen's trial that O'Laughlen entered the home of Secretary of War Edwin Stanton late in the evening of 13 April and enquired as to Stanton's whereabouts. There is still some discussion whether Stanton or General Grant, who O'Laughlen thought was staying with Stanton, was the object in question, but it seems apparent, knowing who else was targeted for assassination, and the fact that O'Laughlen asked where Stanton (not Grant) was, that O'Laughlen was the after Stanton.
Another witness, Bernard Early, testified that the two of them had ridden into Washington from Baltimore on 13 April. He also testified that he waited with O'Laughlen for Booth earlier in the day of the assassination at the National Hotel (where Booth had booked a room), then finally sent some cards up to the room.
O'Laughlen's defense attorney argued that the witnesses at the Stanton home were mistaken in their identification - that O'Laughlen was in fact wandering about the town enjoying the "night of illumination" that celebrated the end of the Civil War. He also argued that O'Laughlen spent the night of the assassination drinking at the Lichau House and left for Baltimore the next day. O'Laughlen voluntarily surrendered himself to federal authorities on 17 April. He was found guilty of conspiracy to murder and sentenced to life imprisonment at Fort Jefferson on the Dry Tortugas, off the coast of Florida. He died there of yellow fever 2 years later, and two years before the other 3 conspirators with him were pardoned.
Much of this information was taken from www.law.umkc.edu/faculty/projects/ftrials/lincolnconspiracy/olaughlin.html.