I think i can say for all of us, myself certainly included, that we don't mean to be harsh, but for the majority of us here a fish is like any other pet - cat, dog, horse, or reptile. while any of those animals could be a favorite or long-time pet, the most responsible thing we can do as pet owners is to meet the needs of the animals we keep as best we can. their lives are in our hands, literally. it is up to us to give them the best 'forever homes' possible, even if those homes are not ours. half of the battle is doing the research in the first place before we even get an animal (and also knowing how to tell the good sources of information from the bad/inaccurate sources) and the other half of the battle is being responsible enough to allow the animal's circumstances to change if we at some point stop being able to fulfill their needs.
while rehoming an animal, ANY animal, can be a difficult thing to do, it is not a shameful thing. it's something to be proud of, that you made the effort to find your pet a good home when it was no longer possible for you to provide one. if you can find your little oscar a new home or even get him a larger tank, pat on the back.
welcome to Aquaria Central. We're here to help.
the first thing you can try is our classified ad section. also register for monsterfishkeepers and see if anyone there will take him. there may be a craigslist for your area, put an ad there. it's free. or register on aquabid and post an auction that is local pick-up only. see if your local fish stores (LFS) will allow you to put up a flyer in their store that your fish is looking for a new home.