If anyone from Oklahoma ever makes fun of you for being proud of TCU's multiple national title-winning rifle team, you can always point out that OU's big non-marquee sport is men's gymnastics- where they've taken home national titles in 2002, 2003, 2005, 2006 and 2008. I use the term "sport" loosely, because when you're performing stunts for judges instead of actually competing directly against your opponents, it's not a sport. That's not to say that I don't admire the strength and athleticism required- and certainly I couldn't do any of the stuff they do- but the only collegiate athletic competition that could possibly rank higher on the "I don't give a shit" meter would be men's figure skating.
The title of biggest non-marquee sport on campus used to belong to the wrestling team, who won national titles in 1936, 1951, 1952, 1957, 1960, 1963 and 1974. It's one of the few sports, however, in which they take a back seat to Oklahoma State- so likely there isn't a whole lot of chirping about wrestling from Sooner fans.