This is a hard question as the Oviraptor was one of the oddest of all the dinosaurs. It is only known from a partial skeleton, so the exact size is uncertain, but it possibly stood somewhere between 3 and 6 feet tall, and 6-12 feet long. It was very bird-like, and stood with the appearance of an ostrich or other flightless bird, but with a longer tail and three sharp hand-claws. Atop a long neck hovered a head with the blunk beak of a parrot meant for crushing, and the odd crest of a cassowary for display. With its hollow bones and relation to birds, it wouldn't be at all suprising if it had a set of downy feathers as well.
An oviraptor
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Tyrannosaurs like Tarbosaurus and Alioramus, and dromaeosaurs like Velociraptor could have been natural predators of Oviraptor.
Oviraptors did, but scientists found a fossil of one sitting on a clutch of eggs which looked like the eggs from Protoceratops . They were actually Oviraptor eggs. The scientists thought that Oviraptor died when it found the eggs
No
Oviraptor, by scientific guess, lived around 3 years, maybe 5 if there are few predators. Oviraptor could well defend itself, though. It is probable they could live longer, but we may never know, due to the fact that Oviraptor is extinct.
on earth
The dinosaur name oviraptor originally meant "egg thief", because of the fact that an oviraptor skeleton was discovered "stealing" eggs from another dinosaur's nest. But recent evidence actually shows that the oviraptor was just being a good mother and protecting her nest from egg thiefs, but most likely a sand storm.
Oviraptor probably would have run from predators. If it was desperate, it could have attacked by biting them with its beak or attacking with its claws.
The Maiasaura and the Oviraptor did.
about the same size as a ostirch
Most theropods were carnivorous. However, some, such as Therizinosaurus, were herbivorous, and others like Oviraptor and Ornithomimus were omnivores.