At some stage in your Oscars life, normally when it is approaching maturity you may notice a strange lump appear on the Oscars bottom lip. These strange protrusions can range from being a very small pimple, to a very large lump. I couldn't honestly tell you exactly why they develop these lumps but it seems to be something that Oscars develop when they approach maturity. It does seem strange that this is the part of their mouth that comes in contact with rocks when they clean them which could possibly have something to do with this.

Sometimes they do look quite sore as they may turn red, or even look like they have erupted open. From my experience, these strange abnormalities are very common and should not worry you unduly. However, if infection does look like it has set in, you could try medicating the tank. However, I'm very much in favour of contacting a vet if the fish looks like it has a serious infection so bear this in mind.