Well, the very best way is to have someone with some open seats fly you up there from Waukegan, Palwaukee, etc.
However, failing that, it's a fairly easy drive from O'Hare (ummm, we do drive on the right, BTW). Just be sure to reserve a car fairly early - even though Oshkosh is far enough away from the Chicago markets where car rentals aren't completely swamped, you won't be the only one doing it that way.
There may be some options via bus, but, depending on where you're staying, you may want the flexibility of having wheels at Oshkosh.