Sock color aside, Ohio State is in b/c they beat Wisconsin 59-0 in a championship game. If TCU played Baylor in a B12 championship game last Saturday the winner would be in instead of Ohio State.
Big 12 petitioned the NCAA for a waiver of the "no championship game unless you have at least 12 teams" rule in the past. I'm sure they will "re-submit" their petition. B12 has been mismanaged for a long time. The UT dominance (of the behind the scenes power and money structure) has pushed Texas A&M, Nebraska and Missouri to other conferences. UT's "Longhorn Network" agreement with ESPN instead of a model like the SEC Network supporting the entire conference is pushing the B12 to the back of the pack of the Power 5.