Oscars are VERY territorial... Almost to the point that they CANNOT be homed with any other fish... usually an Oscar is okay with a Pleco, one that isnt small enough for him to eat.
Like already mentioned they are pouty fish... and if they've been fighting I would suggest maybe separating the two, even if its just until a bigger tank can be bought. Watch for fin bites, if the smaller Oscar has taken a hunk out of the bigger ones Fins he may need some doctoring.
I actually had an Oscar bite the whole underbelly of another Oscar.
Stress is probably why he's sulking... You could try some StressCoat in the Tank.
And just FYI, this sized Oscar WILL eay ANYTHING that he can fit in his mouth...He's a gorgeous Oscar (well, as pretty as a fish can be!) Your MIL is lucky he's still so healthy! All my ex's Oscars ended up with Hole In The Head disease....
Patrón - American Bulldog - 1 year old
Jackson - Bull TerrierXShephard - 1yr 1/2 (DB's dog)
Foose - Black Domestic Shorthair Cat - 2yrs old
Mako - Black and white DLH Cat ( found at work one day )
Expression - National Show Horse - 12-15 yrs old.
"The best kisses come from my puppy "
"There are NO bad dogs, Just bad Owners "