Why is the University of Oklahoma called “OU”?

The story starts with Mizzou, originally the University of the State of Missouri. They didn’t go by USM but rather MSU. They were the first to do this backwards. MU and “Mizzou” quickly became interchangeable with MSU as soon as the 1880s. By 1890, OU and had a charter and MU had a football team. By 1901, MU changed its name to “University of Missouri.” Branding wasn’t what it is now so MU was official but MSU stuck around another 10-15 years interchangeably and Mizzou never left. All teams invited to the Missouri Valley Confrence adopted the same nomenclature.
By 1919 Oklahoma was finally invited to the MVC. They floated the name OK U as a way to fit in, but in the end that wasn’t good enough. They eventually were forced into OU by the 30s.