O'Hara's favorite co-star over the years was The Duke, John Wayne. Though they only appeared in five films together and their relationship off-screen was uniquely a friendly one, the duo became so popular that many in the general public believed they were married in real life. In their first film, John Ford's RIO GRANDE (1950), Wayne plays a U.S. Cavalry officer and O'Hara plays his estranged wife, who shows up at the post one day because their son (played by Claude Jarman, Jr.) is among the new recruits.
My favorite of the Wayne/ O'Hara films, John Ford's THE QUIET MAN (1952): The story of an American boxer (Wayne) who returns to his native Ireland and falls in love with the local spinster (O'Hara), but must learn the local wooing customs before he can have her, and then once he has her, must tame her wild Irish temper before he can find peace in Innisfree. Besides being a good romance, this movie is also a lot of fun, with memorable contributions from supporting characters like Barry Fitzgerald, Mildred Natwick and Ward Bond. It was nominated for Best Picture and won an Oscar for its cinematography.
MCLINTOCK! (1963) is a western taming of the shrew (O'Hara) and a lot of fun. Wayne plays George Washington McLintock, a western cattle baron, and O'Hara is his estranged wife who returns home to get a divorce. More of a comedy than a western, Maureen did most of her own stunts in this film, including a fall off a balcony and an afternoon bath in a giant mud pit.