Yes, I was thinking about Eheim Canisters. I have seen them in the shopping mall near my house. Actually I raised the oscar when he was just 1.5 cm (wild caught). So it was a sudden jerk to update a 20 gallon to 55 gallon since I live in my college hostel.
Sorry, but my experience with canisters has been that are mostly just a septic tank. Most people dont do nearly enough regular maintenance on them, and as the garbage collects in there and decays, it all goes back into your tank as what is now a liquid. They absolutely do a good job of filtration if properly maintained.
And as Carp said oscars are big messy fish, that require more attention to water quality that most, bu to each his own.