[redpaulhus]

Setting up a lifetime home for a fish is no different than setting one up for a rabbit - you plan from the beginning to :
a) keep the animal for its natural lifespan
b) allow the animal to grow to its natural size in a healthy environment.

I'm assuming that you would not setup a hamster cage and then go on a board asking about putting a rabbit in that cage.
Why do so for a fish ?

A good rule of thumb for pretty much any fish is that the tank should be at least 2 "fish lengths" wide and 4-6 "fish lengths" long - so for a 10-12" (25-30cm) fish I would recommend at least a 48 inch x 18 inch tank - and 72x18 or even 48x24 or 72x24 would be better.
Would you honestly put a 1" (2.5cm) tetra in a tank that was 1" wide and 3" long ? Thats the same proportions an oscar in a 45g tank would be faced with.
A single oscar masses about the same as hundreds and hundreds of tetras - would you put 200-300 tetras in a 45g tank ?

I've got an oscar at home who is at least 12" long and 3" wide - he gets weekly water changes in a 125g tank and if I miss one, he starts to get hole-in-the-head.

You don't want to know how many phone calls I take every week asking me if my LFS can take a big oscar that's outgrowing its 55 or even 75g tank - and we generally say no every time (we don't have anyplace to put them, either).

If you can't care for an elephant, don't buy one. If you really want an Oscar, and take proper care of it, you'll have a pet for decades.
Mature, responsible pet ownership is about meeting an animals needs from the moment you buy it until the end of its natural healthy life.
None of us here are ever going to tell anyone that they should be immature or irresponsible about the health and well being of their pets.