If you know for sure that you want to go to OU, then I suggest you wait and rush there.
If you're not sure, I suggest you go through rush where you are and see what happens. You may conclude that none of the sororities at your school is right for you - or you may find some great sisters. You can always decline a bid if one is offered.
If you join a local sorority, you do have the option of joining an NPC sorority later on. I think there's someone on GC who joined a local while in college and later pursued alum initiation into an NPC sorority. It depends on your comfort level with pursuing initiation into a national after having made vows to a local. Just a thought.
