I would put my bets on omnivore/carnivore. However, the closest to an evidence of Oviraptor's diet is a Velociraptor hatchling (or embryo?) found in its nest. So we might be quite sure that it robbed other animals nests, after all (eggs included, of course). Therefore, the more appropriate name would be "Nest-robber". Nuts and mollusks are still a speculation deducted from the shape of the beak and a pair of peg-like teeth.
By the way, doesn't it look a bit too much coincidence that its contemporary Protoceratops also had a pair of peculiar teeth (though of a different shape) in its upper jaw right behind the beak?
Beri's Dinosaur World
"Where were you last night?"
"I went to the movies."
"No, there was about five hundred people."