I'm sorry to hear about Michael O'Hare. I saw O'Hare speak at the San Diego Comic Con B5 panel shortly before the series proper premiered. He was funny and eloquent.
His performance as Sinclair is one of the things that drew me into BABYLON 5, especially in the pilot. While O'Hare wasn't a spectacular actor, the role and his performance in it was quite different than the typical SF-hero at the time. There was something almost Robert Culp-ish about both O'Hare and Sinclair that I quite enjoyed. Unlike Sheridan, Sinclair was a hero who had yet to arrive, who didn't have all the answers, and had a long-buried rage waiting to be unleashed. Which is why O'Hare played him as a bottled-up character, that was often mistaken for being "wooden."
I was disappointed to see him leave the show after one season. I very much enjoyed his performance on the series. EDIT: Someone just posted this on youtube: