The 3-way tie in the Big XII South will be determined by tonight's BCS standings, since they were all 1-1 in head-to-head.
The highest team in the BCS standings will face Nebraska, unless the team 1 spot below them in the BCS Standings is also in the 3-way tie and beat them. That little rule change was added after Oklahoma snuck in over Texas a couple of years ago after losing to Texas and then running up scores.
So 2-loss Oklahoma should face Nebraska, since I don't think 3-loss aTm can get to 1-spot below the Sooners in the BCS Standings. And Okie State should definitely be below Oklahoma.