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I was wondering if anybody knew how big a blue-gill fish would get.  They are native here in Indiana and I was wondering if my O would get bigger than him and if they could cooincide nicely in a 75 gallon tank.  and I think that they are very agressive? so would this work?

Hi, Samanthia!  Bluegill, as you probably know, are a sunfish, and are these days ubiquitous throughout most of the country due to intoduction.  They do not grow as quickly as Oscars, and generally prefer their water a little cooler -- they'll live just fine in Oscar-friendly temps, but probably will not spawn.  They have similar food preferences, and can attain roughly similar size.  They, like Oscars, can be territorially aggressive once sexually mature.  I have kept other sunfish  with Oscars, though not bluegills.  I do know of someone who had a large bluegill that destroyed everything he put in the tank with it until, as a last resort, he put in an Oscar of similar size.  Amazingly enough, they acted like long-lost buddies, and turned out to be great tankmates.

A word of warning, however.  Experienced keepers of Oscars (and other fish) will testify that a lot depends on the individual fish.  One anecdote like the above can be quickly and violently contradicted by your own actual experience, so if you plan to try this, have a Plan B ready in case it doesn't work out.  Also be sure to put any new fish in a quarantine tank for a few weeks to guard against introducing disease or parasites.