You're probably right about the new setting. With fish, it's often not whether or not they've been together, it's "have they been together in this setting?"
It's kind of like with bettas. They'll get along with cories just fine. But if you let a
betta establish itself in a tank and then add the cories, the betta will harass the cory. The answer? Just take the betta out for a little bit while you add the cories, maybe move a bit of the decor around, and put the betta back.
Anyway, your oscars should get things figured out between themselves.
(Also, I don't know much about Oscars. I suppose it could be some sort of mating dance, if one's a male and one's female. I was just comparing it to what other fish do, and the mouth lock is often a form of wrestling for them)