Your oscar fish is laying on its side because of stress or illness. Your oscar might lay on its side after being transferred to a new tank or if it feels uncomfortable with its surroundings.
Oscar fish also lay on their sides as a display of submission if they have just lost a fight or feel unable to defend its territory from intruders.
On rare occasions, your oscar fish might also be laying on its side because it’s sick. If your fish is also showing symptoms of illness, such as gasping and lack of appetite, check and treat it immediately. Also, be sure to check the water quality, as well as the ammonia and nitrite levels.