That protrusion from its vent looks like a prolapse. This can happen through passing eggs or infection. I'm guessing your Oscar may have picked up something from your feeder fish.
That water looks awful milky too, sign of an uncycled tank. As previously mentioned, this "time out" period you are giving yourself needs to be spent swatting up on the basics of fish keeping. The most important area by far is understanding the nitrogen cycle. Failure to get your head around the basics will lead to many more painful experiences such as this one. Good luck, and don't give up, it's part of the learning curve.