Having read a bunch of stuff about Oscars, and owning one now...I don't know what to think, honestly.
See...here's my problem:
1) LFS says 30 gallons are fine for an Oscar... Now, before you flame me...I know that's not true. I can't imagine keeping mine in a tank that small.
2) I have a book that says a minimum of a 55 gallon tank. I'm going with this, by the way... So...flame on, but whatever. There's only him and a pleco in there. And...once I move...that tank will be getting MUCH bigger anyways.
How much is too much?? My two fish aren't that old, I know that. My Oscar is 2.5in, and the pleco is 5in. So...they aren't adults yet. I realize this. But, honestly...you can find anything on the internet stating anything. I have a friend that has a pair of Oscars and they do just fine in his, book recommended, 75 gallon tank.
Now...understand...I hate overstocking, love overfiltering. I don't consider my 55 overstocked with two fish in it. I don't consider my 20 gallon overstocked with 3 tiger barbs and a crayfish. They're both understocked and I like it that way. Now, will they remain that way??? I don't know. It depends on how big the fish. Am I planning for them to be in those tanks for the rest of their lives??? NO, I'm not. What I'm dealing with now is what I can put, reasonably, in my current apartment, so that my fish can be happy and grow. Ok, my Oscar will get to 8in in a year. He still won't outgrow my 55 gallon tank in a year. Then....he might decide that the pleco is a nice snack...I don't know.
I guess what I'm saying here is: Get the biggest tank you can get for the fish you want. My Oscar is my favorite fish, and I have wanted one since I was 9, and saw my first one in 30 gallon tank.
I grow cautious of taking a lot of advice from a lot of different places, because...in order to satisfy every single recommendation I have gotten for my Oscar, I would have to go out and get a 300 gallon tank today, because...he won't be happy in anything less. Well...I don't know about that. The pleco leaves him alone, and I don't, so he seems pretty happy, and spoiled rotten, to me. And I'll continue to breed ghost shrimp for him in a 10gallon tank for him, so he has a snack.
You can flame me all you want, but I think a 55 gallon is fine for now. Maybe you'll need a bigger tank somewhere, maybe not. Only time will tell that.