While the tank size is important, and I do agree that a 55g tank is too small as is the 75g/90g, filtration is most key for keeping oscars. You can always upgrade. Financially it will be a burden, but if you want to "hear the music, you must pay the band". Same is for fish keeping, if you want to keep fish, be prepared to fund your hobby.
I know both plecs and oscars get large, if not huge. Large plecs, however, seem to get very lazy. I once had one that was over 20". I had to poke him from time to time, usually when cleaning the tank, just to see if he was alive. He became more of an ornament than a part of the cleaning crew. Oscars on the other hand remain relatively active. If I were to suggest a tank for an adult
Oscar, it would be no smaller than the foot print of a 300g and heavily filtered. But, we all cannot afford to have such a tank, either financially or due to space constraints.