Registering for classes
Double-degree students receive first priority for registering, then single-degree students.
Because Oberlin isn't a large university, chances aren't too high that your class will run out of space; and if it does, just show up on the first day anyways, as recommended below.
A word of advice for the first-years
Don't stay up all night to catch your 6:30 am registration window on PRESTO unless you already have the alternate PIN (a.k.a. RAP) that your advisor gives you. You need both your regular PIN (the one that used to be your birthday, but you changed it, right?) and the alternate PIN to add or drop classes.
You might not even want to bother staying up all night anyway. It's not really that hard to wake up at 6:30, and you can just go back to bed afterwards. Besides, the classes you want are probably all full anyway, so you'll end up signing up for classes you didn't want. If you show up on the first day of class, you might have a good chance of getting in anyway. Unless the class you want is Hatha Yoga, which you probably can't get into until you're at least a second-semester Junior.