Do Oscar Fish Sleep?
People often think fish don't sleep because they don't close their eyes, obviously no fish has the ability to close their eyes. However, fish do sleep, but it's more likely that fish somehow go into an energy-saving state that could be classed as sleep. But resting or sleep, fish don't sleep the same as mammals, that is almost certainly the case. You'll probably find that Oscars in the wild find somewhere to hide at night, maybe under rocks or amongst tree roots, obviously they want to avoid becoming a tasty meal for the large catfish that are very active during the night time.
Aquarium Oscars will always be active during the hours of daylight. Once the tank is in complete darkness they will settle down. Oscars may float motionless in the water, or rest on the bottom of the tank. Some people leave the tank lights on all day, I would advise that you give the fish a chance to rest during the night so turn the lights off during the night time. It may be a case that fish do actually rest during the day, sometimes Oscars will sit motionless, often leaning to one side.
If you keep nocturnal fish such as catfish or plecos then you may find they disturb the Oscars during the night, you may well here noises and banging coming from the tank at night. One thing to bear in mind is that plecs can sometimes take a liking to the slime coat on fish, they have often been known to suck the side of an Oscar during the night time when the Oscar is resting. If you notice round sores appearing on your Oscar then you may have to remove the plec.