Also, OU (the university) realizes that they won't get this sweet deal in any other conference. While they have a great athletics program, outside of that, what they have to offer is relatively minimal.
They aren't an outstanding academic institution, they own no major markets, they share a small state with another P5 school, and said school has been attached to them in every potential move that they've been a part of. On top of that, Oklahoma State is one of four P5 public schools that carry a "Tier 2" designation. (Alabama, Auburn, and Mississippi State are the others).
Oklahoma's path out of the Big 12 is really, really difficult, and there aren't many avenues for them to go. The Big Ten and Pac 12 already said no. The SEC is their only likely option, but they aren't taking Oklahoma State. So, if OU were to try and make that move, it would be a fight unlike anything we've seen thus far in realignment.