Tank is way too small for an Oscar and therefore too small to add anything else with it.
Sorry but Oscars need bigger tanks. Clicking on the highlighted Oscar in the posts will lead you to the profile in our profile section, there it lists the minimum tank size for them, which in this case is 75g. Unless you can provide a bigger tank for the Oscar, then it will have to be removed otherwise it will simply be very unhappy in that tank, open to disease from a weakened immune system and wont have a very long life span. They grow fast so this must be taken into account.