Are you sure you can't fit a 55 in your room? You can usually get a starter kit with everything you need for about $150, and that's assuming you don't find a used one for less. You could even rig a double deck stand to be a duel TV or stereo stand if space is an issue. Just watch the cords. A 55 is big, but not nearly as bad you might think. And I agree that a 55 will do for a fish that size. And most of the posters here are way too harsh. This is supposed to be a place where those who are new to the hobby can have their questions answered, not a place for them to be rudely criticized and attacked.
An oscar has no business in a 55g tank... sorry, its just simply not large enough. Even if over-filtered and huge water changes are done, it doesn't provide enough room for the fish to comfortably turn around. Could you imagine living in a bedroom 2' wide x 18' long? You'd go nuts knowing you couldn't even stretch out unless you were facing the length of the room.
I believe you are a little quick to deem those who are trying to help here as argumenative, and being too harsh. Yes, we all have strong feelings for these fish, and we've all seen far too many animals suffer due to inadequate housing conditions. This is why these people are so passionate in suggesting any and all means by which to care for the animal in question. Let me modify the hypothetical living space question once again. Would living in a room with a floor plan of 4' x 10' your entire life be satisfactory to your enjoyment and happiness? You'd feel trapped pretty quickly.. that's not even the size of a jail cell. Its not just about what a fish could barely fit into... its about what it needs to be happy and thrive. Just because I could physically stuff an adult oscar into a 6g tank, certainly does not imply that it would be healthy nor happy in said tank.
To the OP of this thread, assuming they come back to read what has been said and comment, I genuinely believe everyone on this forum has the best interest of you AND your fish at heart, and I agree that a 20g tank is in no way adequate for an oscar of any size, let alone at 8" long. This is one of the greatest reasons AC is here, its a huge comminuty of people who have the experience through trial (and sometimes error) to be able to answer most any question you could ask about fishkeeping. Please don't take it personally if the members of the forum are upset about hearing about an 8" 3 year old oscar in a 20g tank... that is effectively comparable to hearing about a german shephard being raised in a 2' L x 4' W x 3' H box.