USC losing/winning probably doesnt really matter, the Pac 12 gets 1 auto invite regardless...
USC losing effectively eliminates whoever ends up #12 from getting a potential at-large spot, since the Pac-12 rep will almost assuredly be outside the top 12.
USC winning may move them into the top 12 (since they were #15 before beating UCLA), which could leave #12 in the at-large pool.
If you're looking at worst-case scenario (losing to OU), then it's all about keeping #10-12 in play.