If you have another empty tank then when you do a water change, use that water to fill or part fill that tank. Move across one of the filters too if you have more than one. If not you need another filter and can move just some (half) of the filter medium from your existing filter into the new one. Then refill the old tank with new water and add around the same volume of water you already added to the new tank too. This way both tanks are now set up with the same water and a working filter. The Oscar can be moved into the new tank.
Note however that you are not giving me a warm fuzzy feeling that you are testing the water and doing good for the fish. Not knowing the water results for a tank you have obviously had running a while and then Mentioning he is always fighting 17 posts in to a thread asking why he is not eating is not good- this is no doubt why he is not eating , because he is being bullied.