Betsy is most known by her role as the main female lead, Penny Lace, in the Vinewood clasic film, Bullwhip Fury. Betsy was considered "America's sweetheart" in 1952, despite accusations that she engaged in sexual relationships with movie directors and producers to get some of her roles. She has been referred to by some people as "the Poppy Mitchell of her time".
O'Neil was one of the actresses considered for the female lead in the 1975 film The Many Wifes of Alfredo Smith but at 40 was considered too old for the role. Leonora Johnson, a much younger actress, was also considered for the role before she was murdered. This led to suspicion that O'Neil had murdered Johnson as an act of revenge, until it was later revealed she was murdered by director Peter Dreyfuss.