speaking in my own .02 and exp,i would get away from the oscars, the grow really fast and you will be forever upgrading, not only that when they get to be about 8" if you try to intruduce new fish the better be healthy big and quick, as the O's like to just swallow anything new in sight,
anyway, has anyone asked, what sizes are your current fish?
i made a lil chart to keep track of mine. amount of, fish name/type, current size, adult size, and a total, most say 1" per gallon MAX and from my exp the closer you get to that limit the more you will be fighting water problems, i personally agree.
if you are already over that rule, i would definatly consider the oscars to leave first, they do need much more room than most, HOWEVER i do not beleive you need 210 gallon tank for 5 fish. any fish. if pets outgrow you need to upgrade or trade. even if you dont get much for them they will be better off, and this is all my own opinion. subject to change anytime, lol.
my tank is currently 13 total at 18" total, however at adult it will be 54"-60" (approx) and i plan to add two more so i will be right at my rule.