Here's my 2 cents...
1. I just want a big plant, do I need to separate or can I just keep getting bigger containers? You can keep getting bigger containers. Our OS president had an enormous, enormous specimen plant. I think the main issue is to manage to get the plant throughly wet, but let it drain quickly and thoroughly as well. I find oncidiums a bit tricker to water than catts. I do know that some folks with specimen size plants put an upside-down net or other pot in the middle of a larger pot...so that you don't end up with a big wet center than doesn't ever get to dry out.
2. What type of artificial lighting are you using on your Oncidiums? Mine have been indoors on plant shelves with lights all winter and seem happy (and bloom). They spend the summer on the patio in a semi-shaded area.
3. Oncidiums aren't mentioned as much as Phals and other orchids, so people not "like" them much? I think some do, some don't. I like them, but I do find the watering to be a bit trickier.
4. I read that they will climb out of the pot or over each other, what is the best container to grow them in? They do tend to climb. I grow mine in plastic pots, and don't worry about it.
5. When people say they add fertilizer to the mister are they misting the leaves or just the roots? I mist whole plant when I do this...foliar feeding. It is in addition to regular fertlizer though.
This is only my experience though!