Three negatives of going to the SEC:
1. Leaving behind the opportunity of gaining an advantage on the West Coast recruiting grounds—Let's face it. There are really two options at this point. OU can join the Pac-10 and build a solid base on the left coast for recruiting, or it could improve its stock by becoming a part of the SEC, which is so competitive. If the Sooner program decides on the latter, they won't be playing against those teams on the West Coast on an everyday basis, and therefore they will fail to gain a stranglehold on that region of the country. However, the recruiting territory gained in the SEC region could easily make up for that.
2. Potentially losing conference ties with Texas—At the surface for Sooner fans, it might not sound like such a big deal. These two always found a way to play every year when OU was in the Big Eight and Texas was a member of the old Southwestern Athletic Conference (SWAC). But it will be harder to justify scheduling that game each season if OU played in the tough SEC. A part of what OU loses by not being in Texas' conference in terms of strength of schedule they could gain with all the other challenging opponents. So, they could certainly lose conference ties if the Sooners traveled to the SEC and Texas went elsewhere, and they potentially might not play the rivalry game every year.
3. OU would have to adapt its style of play some—Again, the Sooners have historically been an excellent defensive football team with a great ground game, many times featuring the option and heavy use of the ground game offensively to control the tempo and grind it out. But in recent years, OU has moved more towards the air attack with the pro-style offense featuring gun slinging quarterbacks and speedy wide outs. In the SEC, it is a speedy, ground attack style of offense. And at the current time, OU, while it does utilize its running game efficiently (i.e. two years ago when Chris Brown and DeMarco Murray both had 1,000 plus yards), uses the air attack a lot more than the SEC. Do I think it would be that drastic of a change? No, but this is a potential negative.
Check here for the potential positives of a move to the SEC.