I don't know about Oscars but my Angelfish do something of the sort too. They sometimes seem to be "yawning" too. I don't know what it means but it could be that they're simply stretching their muscles, maybe? My angels do that often after eating - maybe they're trying to, sort of, "push" the food down their gastric tract in order for the food to get digested faster so that they could swim at ease? Just my thoughts. If my angels do that, probably all fish do. And I think it is normal and there is nothing to worry about. Unless they're "yawning" at the water's surface - which could be gasping for air.
thanks you guys probably have nothing to worry about,I'm new to this hobby(accidently)and learnt to love and appreciate my fish.I'm always courious,found out from gunnie that I definitely need a bigger tank for my oscar,so I'm left with my 20gal.tank with 1 pleko and 1 parrotfish could I put another fish? if so,what??
My goldfish do it all the time - iot makes me laugh because it looks like they're yelling at each other under water heehee. I think they're either moving food struck in their mouth or cleaning their gills or something.. its pretty cool tho.
I'm not sure if they are really yawning I think they are trying to inhale some food particles from the water, cause they come from the Amazon right? so it could be a natural instinct, I mean the amazon river was never the cleanest looking river so maybe food was drifting through the rive frequently and that's how the habit started.
Adding on to everyone...Oscars [and all fish] do yawn. Its perfectly normal. My fish do it all the time around nights when its most frequent. I have two oscars and have been in the hobby for 4 years now. Dont worry about a thing.