I would get the tank first, then get some other fish. In a 180, a nice group of silver dollars would be awesome. Just need to remember anything that can fit in an oscar's mouth is a meal to them. And when fully grown, they can carry a golf ball around in their mouth.
Go with a big tank. I kept keeping a minimal size tank and having to upgrade every so often. I was young, and broke, but Oscars are what set the addiction for me, and some of my favorite fish to be had. I just don't have the space for such large aquariums. Plecos and silver dollars are what I kept with mine. If it can fit into their mouth they will eat it.