On September 28, 2012, J. Michael Straczynski posted that O'Hare suffered a heart attack on September 23 and had remained in a coma until the 28th, when he died.
Very sad to hear this. I was and still am a huge fan of the 'Babylon 5' series and I really enjoyed Michael O'Hare as Commander Sinclair. I remember when season 2 started being devastated that O'Hare would not be returning. I had no Internet back then and had no way of knowing Bruce Boxleitner would be replacing him on the series. He did make a brief return appearance in Season 2 but made a big splash in the 3rd season 2 part episode 'War Without End'. I had always wished they would have had him back more on the series, especially when they started doing TV movies. I had wanted them to do a 'Babylon 4' movie which would have focused on O'Hare as 'Valen', who was actually Commander Sinclair transformed into a Minbari.
Rest in Peace Mr O'Hare, you will be missed and never forgotten.