[1] From Zamboanga City, the O'Haras moved to a middle-class suburb in Pasay; behind their house was a slum area, and Mario claimed that some of his works were inspired by real-life incidents that happened there.
At the age of 17, Mario stopped attending classes in 1963 to focus on his work in radio drama with the Manila Broadcasting Company (MBC) and DZRH.
He directed the drama series Lovingly Yours, Helen, Flordeluna (featuring Janice de Belen) and Alitaptap sa Gabing Madilim.
Brocka offered O'Hara a role in his 1971 film Tubog sa Ginto ("Goldplated) and also cast him in dramatic productions at the Philippine Educational Theater Alliance (PETA).
O'Hara, in turn, wrote the screenplay for Brocka's Weighed but Found Wanting (Tinimbang Ka Ngunit Kulang, 1974), about life in a small provincial town; in the movie, he also played the role of Bertong Ketongin (Berto the Leper) vis-a-vis Lolita Rodriguez, who played Berto's love interest Koala.
During the 1980s, O'Hara collaborated anew with Nora Aunor: Kastilyong Buhangin ("Castle of Sand", 1981), featuring Lito Lapid; Bakit Bughaw ang Langit?
The first pito-pito was the drama Babae sa Bubungang Lata featuring veteran actress Anita Linda and set at the Manila North Cemetery.
O'Hara's 2004 film Babae sa Breakwater addressed issues of poverty in Manila and was met with some considerable success.
[2] He was admitted at the San Juan de Dios Hospital in Pasay, Philippines, on June 15, 2012, to seek treatment for leukemia.