I'm glad your puppy is better. I have a very strong disdain for people who litter. It seems like a petty crime but in my view a person who does not care about or has no ability to care about our environment is poison to the planet. Enough said, back to topic,,,
Your oscar. They do not get "pregnant" per se. When they breed the female produces ova (eggs) and becomes a bit rounded in the belly. She then lays them and the male fertilizes them nearly simultaneously. It is unusual for a female oscar to produce ripe ova without the stimuli that set it off, notably the presence of a male and also the environmental triggers.
Without seeing your fish I am making assumptions, but I think if you are seeing a notably distended abdominal feature it is probably not related to egg production. Even under the best of circumstances when a female oscar is about to lay eggs there is usually only a cursory amount of swelling around her belly just before egg laying.
What is it then? It could be distended from a big meal. Did a family member recently dump an inordinate amount of food into the tank? A child dumping food in perhaps?
Also it could be distended from a bacterial infection. Again, pure speculation.
If you post a pic it might help. Also, the age, size, how long you have had it, all that pertinent information that may help.
Bill