Very sad news. That's now four main cast member deaths for a show that started in 1993. B5 must be fast approaching some sort of unfortunate record of premature deaths.
You're not the only one who thinks that way. From FB: I never met Michael O'Hare but very much admired his work on B5. Jan
I remember a while ago, J. Michael Straczynski was asking if anyone knew how to contact him. While JMS kept the reason why to himself, many speculated it was due to Michael O'Hare's declining health. Sad to hear of his passing. I really liked his portrayal of Commander Sinclair and will defend his acting in Babylon 5 every time. Sinclair was supposed to be played as somewhat wooden and Sinclair captured that.
I liked Sinclair a lot and I liked the way that O'Hare portrayed him. I'm very, very upset to see that he has passed away. What a complete bummer.