He was FUBAR long before that.
O'Hare supposedly had Bechterew's disease, along with visible signs if you recognize those. His vertebrae had already begun to fuse by the time he was cast on B5. That's why he was on loads of painkillers and sleeping pills. He also suffered from asthma. He had to give up tons of roles because of his poor health. (As a result of this, there were rumors swirling around about drugs. He got a lot of undeserved shit online, and Jerry Doyle poured fuel on that fire.)
"War Without End" was shot when O'Hare got in good enough shape to resume filming. He was on very strong meds, and the side effects showed.
Of course, the treatments were never entirely successful. He had visible tardive dyskinesia by '98 and gained plenty of weight.
If watch old convention videos, it's easy to see why he could have been a success. Sort of a marriage between Shatner's looks and Stewart's delivery.
https://youtube.com/watch?v=0pR5IuVptRw