She was born Mary Margaret O'Shea on October 3, 1922 in Niagara Falls, New York, the fourth child of Thomas and Francis O'Shea.
She initially broke into TV with her ex-husband, Lou Shaw, on a range of other including Dr. Kildare, Ben Casey and Have Gun Will Travel.
O'Shea served as head writer for daytime soap operas One Life to Live (1979-1983, 1984-1987) and Capitol (1983-84, replaced by Henry Slesar). She also wrote for primetime serial Peyton Place.
She won a WGA Award and a Daytime Emmy Award for her work on One Life to Live. She was nominated for four WGA Awards and five Daytime Emmys in total.
O'Shea lived in New York for years during her tenure at One Life to Live, but her heart was always in Los Angeles.
O'Shea is survived by Survived by son Christopher Shaw, grandchildren Ari, Emma, Caius, and Courtenay and daughter-in law, Danika.