I never knew the reason O'Hare left the show, perhaps it wasn't openly discussed at the time? At least his character arc got wrapped up later.

There was a lot of behind the scenes drama during the fourth season, mainly with the outfit actually syndicating the show. That put pressure on to wrap things up. The final episode was actually filmed in season 4, but held back when the last minute reprieve came, and the show moved to TNT on cable. (One reason why season 5 feels like padding.) TNT would later meddle too much on the sequel series Crusade.

I'm still disappointed Garibaldi never got revenge on Bester. They should have at least done that in one of the tv movies.

If you want to have a surreal experience, you have to see the low budget sci fi film, Arena. It plays like a B5/DS9 crossover episode, with actors from both shows. Probably helps I first saw it about three in the morning on TNT, when both series were still airing. ;)