I would have thought, not much, but I looked them up and apparently an oblong is any shape longer than it's wide, even a rectangle. Oval is from the word for "egg," so it means, well, an oval. Oblong just means "long."
So today I learned something from usingenglish.com. I'm betting, though, that most people who say oblong mean oval, unless they're mathmaticians or botanists or something.