30g may make a good starter tank, but a 55 gallon tank is bare minimum. Your little guy will grow pretty quickly, so you may want to start saving. With a max size of over 1', you are going to need a tank bigger than a 30g.
Check craigslist. I have seen full setups (w/ 55g tank, stand, filters, heater, decor, etc.) for under $100 in my area. Just make sure you get enough information from the buyer to feel confortable with getting the tank. I got my 30g saltwater tank w/ 10g sump, powerhead, 30-40lbs of Live Rock, Live Sand, and my 2 damselfish for $150. The possibilities are endless!
Tiger Oscars are awesome! Can't wait to see pics!