OU had their chance several years ago, and just couldn't bring themselves to cut the cord from UTw.
For some reason -- and I'll never understand why -- their school administration and fanbase felt they couldn't thrive without Texas. While they need the state of Texas for recruiting (the state of Oklahoma not producing enough quality HS talent of sustain a college team at the national level), they felt they needed the University of Texas to accomplish that.
And I've never been able to find a credible judtification why a program with the tradition and stature of OU needs anybody to help them out.
Several years ago, OU would have been a great addition. But lots of things have happened:
1. There was a lot of talk that the Oklahoma state legislature would never allow OU to leave without forcing Okie Lite (Oklahoma State) as a package deal.
2. That was a really dumb move, because they needed the SEC a lot more than the SEC neeed them. Now they have two schools in the Big 12 wilderness, not just one.
3. Since then, the imbalance of power between the Oklahoma schools and the SEC has only gotten more pronounced.
4. The last ship sailed when Mizzou joined.
5. With Mizzou's lack of real contribution in any sport fresh in mind, I doubt Sankey or the presidents would be terribly enthused about OSU's sports programs.
5A. If you take OSU, you also get one of two college programs that have a de facto owner in Boone Pickens (the other being Oregon and Phil Knight). The SEC won't kowtow to anybody, and Pickens won't have it any other way. I don't see that gulf being bridged.
6. Taking OU alone would require a total re-alignment of the conference divisions. No way they have Alabama, LSU, OU, and the barn in the same division. This one actually might not be such a bad thing, but it could disrupt a lot of traditional rivalries.
6A. Compounding the re-alignment issue, you'd have an odd number of teams. You either take a 16th team (if so, who?), or you live with the logistical nightmare of 15.
As I said, this is an idea that might have worked a while back. But its time has passed.