Oscars,when properly cared for,average around 1" a month until they reach 11"-12",then slow way down with any additional growth being thought of in terms of years. So a long-lived Oscar is capable of 15"-16".
They aren't particular about decor ... rocks/driftwood is fine,plants (unless something like Anubias or Java Fern) will more than likely get ripped out.
If you keep filtration at it's best,and keep water quality in the ammonia 0/nitrite 0/nitrate <10 range with appropriately sized water changes on a regular schedule,with a varied diet (not just pellets and 1 or 2 "treats") they can live up to 20 years.
Some like "toys" to occupy themselves (I use ping pong balls).
A 75g is a great size to start with ... as long as you don't get carried away by wanting to "add". That volume permits some low bio-load dither fish and a medium bio-load tankmate ... and really not much else if going for maximum growth,good health,and a long life.
For varied food ideas http://www.theoscarspot.com/viewtopic.php?f=3&t=13487 is as good as any ... most Oscars will eat at least 90% of what's on the list.
Stay away from "feeder" fish for the best health (google thiaminase... nuff said).
I'd also advise joining a forum ... a specialized forum ... to gather ideas and share info. I'm partial to one in particular,but I'm not here to "hawk" you ... so look around.