I don't have much knowledge of beardie teeth, but I've read that their front teeth can grow back if they are somehow knocked out, however since so many of them aren't visible I can't see how that could have happened. My best guess would be that nothing is wrong, especially since Oscar is still eating live food. Dubia roaches and crickets are much harder to chew than greens, so I think if he was unable to chew his greens, he certainly wouldn't be able to eat crickets or dubias. I can't recall seeing much of my beardie's front teeth either, and she's never had any problems with chewing her food. Also, if you look closely at the photo provided you can see little bumps which could be the teeth. Again, I'm not an expert in this matter but I wouldn't worry too much about it. Like mattmarraj said, Oscar's probably just being picky. You can try giving him some new foods if you have any available, a popular list used for finding foods safe for beardies is
http://www.beautifuldragons.com/Nutrition.html if you don't use it already!