I came across this arowana forum and read an post about fish yawning and I think oscar would have similiarities in this too?

Here we go:
www.indodragon.com/forum/f2/arowana-yawning-sign-relaxing-5323/

Yes that's quite interesting. Sometimes my Oscars do yawn, but I wouldn't say they do it very often. I've always put it down to them either stretching their jaws, or maybe taking oxygen from the water. Some people think that fish that constantly yawn are doing it because the water quality is not so good. But if you actually do some research on yawning, there are loads of reasons put forward why people or animals yawn

This thread made ME yawn! LOL

fish do stretch their muscles like us
see this avatar...

PAUL wrote:

Oh, I allways thought this Oscar was singing "The sound of music"!lol

lol i think mine do it only wen they see their reflection, so i think it has something to do with seeing another O they arent used to seeing

and this is not yawning.... and not reflection either...they are muscling
up who is the boss

i think they open their mouth/tubes for water to get pushed in and help digestion