The last 2 oscars died because 1 died from a power outage a few months ago and the other died because a certain relative thought it would be funny to prank me by pouring food coloring and some other harmful chemical into my oscar tank. But if its necessary, 55 gallon tank with one oscar where I do 25% water changes weekly and my filtration media gets cleaned once a month. The filters I use is a mix of pothos plant filtration and Fluval HOB
If a relative pulled a prank like that on me, they would never step foot in my house again.
Do minimum 75% water change per week for your O on the regular. Do it one extra time this week and he will heal up fine. Amazing what simple water changes will do. Os are massive waste producers and also secrete growth reducing hormones that can stunt them in the long turn. Make sure to do the changes regularly.
Also, only clean your filter media in used tank water, never under the tap or you will use your beneficial bacteria (chlorine kills it). Maybe you know this, but can't hurt to say it again.