Part of it is the shooting percentage of Missouri. As a general rule, the defensive team has an advantage rebounding. Missouri had an opportunity for a lot more defensive rebounds.
Unfortunately, missouri also had more offensive rebounds than OU. That part is really tough to accept because OU should have had more offensive rebounds simply because OU had more opportunities for offensive rebounds.
I think the easiest explanation is Missouri is simply a better team than OU and played a good if not great game.