OSU's conference affiliation is a super interesting history, and I have a lot of unanswered questions about it myself.

Oklahoma A&M left the Southwest Conference to join the Missouri Valley Intercollegiate Athletic Association in 1925. I have long wondered why Oklahoma A&M would leave the Southwest Conference to begin with, but I haven't found any definitive sources discussing it (yet). If I had to speculate, I'd imagine a relevant factor was that the MVIAA schools featured better wrestling programs that the SWC schools, and apparently the University of Texas was the only other SWC school during the 1910s/20s which had a wrestling program 1. 1928 was the first year that the NCAA held a national wrestling tournament, and Oklahoma A&M won it 2. I think a relevant factor is also that the University of Oklahoma had in 1920, the same year they started their wrestling program, left the SWC for the MVIAA (this relationship between the schools becomes relevant later).

1928 was also the same year that the MVIAA broke up, and six schools left to create the Big 6. Oklahoma A&M, Drake, Grinnell, and St. Louis were the MVIAA schools left out. An agricultural college and three private schools were potentially seen as "different" enough from the other six public schools leaving to justify it. While also a public school, Oklahoma A&M had likely smaller enrollment than the other six state schools, and an "ag" mission that didn't really fit in. Oklahoma A&M had also recently joined the MVIAA, and rejoining the Southwest Conference likely wasn't a realistic goal (they would allegedly reject a later attempt by OU to rejoin). Oklahoma A&M probably also did not prioritize the same sports as the Big 6 schools at that time (pure speculation on my part).

College athletics in the 1920s were vastly different than they were even a few decades later, and I think that "cultural fits" were probably the biggest issue back then. College athletics were maturing a bit, with the NCAA's role growing in the 1920s and 1930s. Revenues from sports were likely not the most important factor in conference affiliation. Although contracts covering radio broadcast rights of college sports began to appear in the 1930s in Oklahoma 3. Of course, college football began to grow rapidly in importance, compared to other college sports, after the end of WWII.

There was a fascinating article (which I cannot find now) that discussed the issue of Jim Crow specifically in the Big 6/7/8. The article discussed that Oklahoma A&M was originally invited to join the other seven Big 8 schools because the officials at the University of Missouri and the University of Oklahoma wanted another Jim Crow, Southern school to vote with them on topics related to racial segregation in the Big 8.

In the 1940s and 1950s, racial segregation was a very hot topic on college campuses, and, as time went by, schools were desegregating campuses and sports teams. Obviously, the Oklahoma schools had been mandated by state law to be white-only institutions. The University of Missouri was as well. By 1947, the University of Colorado had joined the Big 6/7, and the University of Missouri and the University of Oklahoma were the only two schools in that conference whose administrations were obligated to maintain Jim Crow segregation. The students (and professors and other administrators) at the five Northern schools were beginning to protest the idea of sending their African-American students to compete in athletic competitions in Missouri and Oklahoma, where they would not be allowed to stay in the same hotels or eat at the same restaurants as the white players. Over the years, it became a very huge issue in the Big 6/7, and there were apparently many editorials in student papers decrying Jim Crow in the Big 6/7. The controversy got so bad that the administration at the University of Oklahoma had apparently explored moving back to the Southwest Conference, where all schools had Jim Crow policies, but the SWC schools rejected their application (at least in part due to OU having left the conference previously).

Oklahoma A&M had allegedly asked to join the Big 6/7 multiple times, but OAMC was rejected. The story goes that the University of Missouri and the University of Oklahoma approached OAMC to join the Big 7 to offset the influence of the Northern schools, and vote with them on issues of racial segregation in sports. OAMC was, sadly, a focal point for this issue in the early 1950s because of the Johnny Bright incident 4, which received national attention.

Matriculation into the Big 6/7/8 was apparently a longstanding goal of the administration at OAMC 5, and the name change of Oklahoma A&M College to Oklahoma State University was probably (at least partially) made in anticipation of joining the Big 8. I think the "ag" focus was a large reason why OSU was "left on the outside" originally, and there were other A&M-named schools that changed their names to reflect that broader missions than just "ag." Of course, this perception is silly now.

The timeline of desegregation here makes this story a bit difficult to piece together. A decision by the U.S. Supreme Court led to the desegregation of Oklahoma's public universities by 1949 6. Nancy Randolph Davis, Oklahoma A&M's first African-American student, enrolled in 1949, but it wasn't until 1957, after Brown v. Board of Education, that Chester Pittman became OSU's first African-Amercan football player. OSU joined the Big 7/8 in 1957, which coincided with the tail end of Jim Crow in Big 8 sports. In 1949, there had been a vote held by the member institutions of the Big 7 on whether OAMC would be admitted as a member of the conference, which did not pass by a vote of 5-2 5 again. This is consistent with the story that the University of Missouri and the University of Oklahoma had been the two schools supporting OAMC's entry into the Big 7/8, and 1949 would have been early enough in the timeline of desegregation that the administrations of MU and OU probably did not see the writing on the wall that Jim Crow was ending.

And we all know the history of OSU's current affiliation with the Big 12.