BCS Rules:
The third-ranked team will receive an automatic berth if it has not already received one, and if it is a member of a BCS Conference.
If the third-ranked team did not require an at-large berth, then the fourth-ranked team will receive an automatic berth if it has not already received one, and if it is a member of a BCS Conference.
Something to consider when looking at this possibility.
Let's say according to your scenario...
Final BCS rankings:
1) Oklahoma
2) Texas
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3) Florida (automatic bid)
4) Alabama (would get a bid according to the above rules)
5) USC
Now, if they fell below 4th, then I still think they get an at-large.