If other fish are pecking at your oscar he is really sick.
When you get to the fish shop buy your own testing stuff. Dont get the strips buy the bottles, you will need ammonia, nitrite and nitrate then you can test at home whenever you want
I am hoping your problem is caused by high nitrates. When i was inexperienced, i never used to do water changes. After a surprising amount of time the oscar i had at the time was all lethargic and looked really odd and moved funny. A friend came round and we change nearly all the water and immediately my oscar looked fine again. I would not recommend doing this now. Do a 50% water changes everyday until you get your testers
Your test results should be ammonia and nitrite zero and nitrate ideally delow 20 but max 40
How big is your tank, oscars need at least 55g. If you are going to feed live fish quarantee them for at least 2 weeks before placing them in your oscars tank