It is fairly common for your Oscar fish to develop a strange lump or protrusion on its lower jaw. These lumps are often described as looking like pimples, warts or even zits coming to a head. As far as I’m aware this lump hasn’t got a scientific name, although I did see the word “chinple” appear somewhere.
I can’t honestly tell you why these lumps appear. However, they do often appear when the Oscar is approaching maturity. As anyone who has Oscar Fish will tell you, Oscars absolutely love to dig around in the substrate as well as vigorously cleaning the flat rock which they often monopolise as their own. Because the surface area needs to be clean, they use their mouths to scrape any dirt or debris off the rock. I believe that this pimple develops because of this reason. I have no scientific proof to back up my claims but many of the Oscars I have had over the years have developed this protrusion in the exact position on their jaw that they use to clean their rock.
I did once have an Oscar that developed a pimple that became very sore looking, almost infected. I decided that I wouldn’t intervene and I just left it alone. Thankfully the pimple didn’t develop into anything horrible and remained on the fishes lip for the entirety of its life. However, if you are worried that a pimple is infected, then you could medicate. If it was me I would probably use something like KRYSTON KLIN-IK, which is an antiseptic medication usually used with coarse fish such as carp, however, it works very well on Oscars and can help to prevent infection. Occasionally you may have an Oscar that develops an absolutely huge pimple that not only protrudes from underneath the lip, but also appears on the inside of the lip as well. Some people have suggested that the lumps that appear to be getting bigger may have been caused by Oscars that are active diggers and also use their mouths to investigate things in the tank, i.e. moving rocks, substrate, picking up bits of wood etc. It is possible that the Oscar could have irritated the inside of it slip and therefore encouraged lumps to increase in size. If this describes you, Oscar, then you could try a removing the objects that they will not leave alone. A more drastic move could be to change gravel for sand. Sand particles are obviously a lot smaller than gravel and therefore the Oscar may reduce the chances of getting irritated when it takes in the substrate. I must add that this is just a suggestion, and by no means an absolute answer to the problem because it may have no effect whatsoever.