Bros he was reunited with Cagney for The Fighting 69th (1940) then made Castle on the Hudson (1940) with Sheridan and John Garfield. He was a cop in 'Til We Meet Again (1940), with Cagney and Sheridan in Torrid Zone (1940). He co-starred with Garfield and Frances Farmer in Flowing Gold (1940). O'Brien was then given his best known role, as the famous University of Notre Dame football coach Knute Rockne in Knute Rockne, All American (1940). In the film, he gave the speech to "win just one for the Gipper," referring to recently deceased football player, George Gipp, portrayed in the film by a young Ronald Reagan. Reagan later used this saying as a slogan for his campaign for president in 1980. O'Brien was at a career peak. He was considered for the role of Alvin York in the film Sergeant York. From this high point, however, O'Brien left Warner Bros in July 1940. Leaving Warner Bros After he left Warner Bros. in 1940, O'Brien signed a contract with 20th Century Fox for two films a year. However they ended up not using him. He signed with Columbia Pictures to make two films a year. He was in Escape to Glory (1940), then was idle for a year before making Two Yanks in Trinidad (1942) with Brian Donlevy and Flight Lieutenant (1942) with Glenn Ford. At Universal he was in Broadway (1942) with George Raft. RKO Soon he signed a contract with RKO and appeared in several movies for that studio. He mostly played authority/military roles such as The Navy Comes Through (1942), and Bombardier (1943). The Iron Major (1943) was an attempt to repeat the success of Knute Rockne with O'Brien as Frank Cavanaugh. At Universal he supported Deanna Durbin in Frank Borzage's His Butler's Sister (1943) then it was back to RKO for Marine Raiders (1944). With his agent Phil Ryan, O'Brien set up his own production company, Teneen Productions. They signed a deal with Columbia to make a film with O'Brien, Secret Command (1944). (In 1955 the IRS would sue him for earnings on this film.) At RKO he did Having Wonderful Crime (1946) and Man Alive (1945). For Columbia he made Perilous Holiday (1946). In 1946 he starred in the successful film noir suspense film, Crack-Up. He was in a thriller, Riffraff (1947) and another biopic Fighting Father Dunne (1948). He followed it with The