Oenslager was born in Harrisburg, Pennsylvania and attended Harvard University, graduating in 1923. He became interested in design while studying in Europe and his first work as designer was for the ballet "Sooner or Later" in 1925.
He had started as an actor in the 1920s at the Greenwich Village Theatre and the Harrisburg Playhouse.[1]
He taught scenic design at the Yale School of Drama for many years [1] and the institution continues to award an annual scholarship for stage design in his name. Among his notable pupils was John Lee Beatty.