The good thing is that you already realize that an Oscar shouldn't be housed for life in a 55. That topic gets argued and beat to death around here. If you have plans to have a 210 up and running in the near future, you can get an Oscar now at let it growout. Tank size, diet, regular water changes are all important with keeping cichlids. We may not be able to duplicate the humongous sizes of rivers and lakes but we still can supply the fish that we choose to keep with a healthy amount of space. I think your future 210 will be great for your mentioned stock.
That should work out fine. What kind of Oscar are you getting? I really like the WC ones. I saw some for sale in the marketplace for $15.
I'm have read the first page of this, and I'm sure I already know what the other 4 pages say. People love to overreact to this topic. Fact is, most ppl who don't know what these fish require and assume they can keep them in a 10 gal or something. You appear to be on the right thought path.
You'd be fine with an oscar in a 55 up to about 8" or so. Start pulling deco out of the tank as he grows (until you get the bigger tank for him as an adult) to make room... oscars are clods, they run into everything.
It took my oscar from 3 inches to 9 inches in 7 months the most,I had the oscar in a 55 gallon with other tank mates and upgraded to a 125 gallon.I must of gotten a pissed off unsatisfied oscar,because this one is always pissed about something or not satisfied even in a 125 gallon.Then again it depends on their personalities,so I am not going to say all oscars are like mine.