O'Brien grew up in New Britain, Connecticut and served in the United States Army during World War II.
[2] O'Brien's balding, mature look helped gain him roles as authority figures, with his son recalling that "he always had that older look" even in his early acting days.
During the 1960s, O'Brien appeared on the soap operas Dark Shadows, The Edge of Night and twice, as two different characters in two different decades, on The Guiding Light as Colonel Grove Mason (1969–1970)[citation needed] and Dante "Pops Cooper" Kouperakis (1987–1990).
In the late 1960s and through much of the 1970s, O'Brien appeared on television and print advertisements as the Shell Answer Man, a job that he felt was "like hitting the state lottery".
[2] During the 1970s he appeared on Ryan's Hope and played a hotel doctor in Woody Allen's 1977 comedy Annie Hall.