He was best known for playing the lead role of space station Commander Jeffrey Sinclair in the first season of the science fiction television series Babylon 5.

He attended Chicago's Mendel Catholic Preparatory High School,[1][2] where he played football, to defy his doctor who told him he would never be in athletics because of his asthma.

[23][24] O'Hare appeared in a number of theatrical productions on Broadway and in regional theaters, including an acclaimed revival of Shaw's Man and Superman[25] with Philip Bosco; in the role of Col. Jessup in the original stage version of A Few Good Men;[26] as Captain Solyony in Chekhov’s Three Sisters.

[27] Other notable roles included Alfred in a 1986 stage revival of Little Murders;[23][28] Jake in A Lie of the Mind[29][30] and John in Lips Together, Teeth Apart.

He did some voiceover work for commercials and read a radio adaptation of the science fiction novella Think Like a Dinosaur for Seeing Ear Theater.

Babylon 5 creator J. Michael Straczynski revealed after O'Hare's death that the actor had had severe mental illness.

Straczynski offered to suspend production for several months to accommodate treatment; however, O'Hare feared that such a hiatus would put the series at risk, and he did not want to jeopardize others' jobs.

O'Hare told him to instead "keep the secret to my grave", arguing that fans deserved to eventually learn the real reason for his departure, and that his experience could raise awareness and understanding for people with mental illness.