As necromancer said, if they are new to this tank this is usual behavior. Nothing wrong if they take more than two days to feel at home. If this persist past some time, say 10 days, then something else might be acting as an aversive stimulus for them.
Oscars' natural habitat is dim in sunlight due to dense major vegetation above the body of water they thrive in.
I use bronze-tainted glass instead of clear standard glass in my canopies. Plants don't get all the light spectrum they might like but still this little change makes them feel really comfortable all around the tank.
My Oscars were introduced simultaneously with one Green Severum, a few days later I brought a Gold Severum. They were all juveniles, the two Tiger Oscars were 6 and 4.5" while both Severum were around 5. I can tell you that the Gold Severum ruled that tank for a month or so. Now, with them at 6 to 7" and the Oscars' at 12 and 11" it's a completely different story.
My Oscars wouldn't even eat in my presence for a while, now they both feed from my hand. We do interact a lot.
Pepe
Santo Domingo