Martyn, what to grow it in really depends a lot on where you are and the conditions under which it will grow. Generally speaking, my oncidium orchids like a bark mixture - but the finer the roots are, the finer the mixture can be. Where I grow them it's so dry that I actually put some sphagnum moss into the bark mixture, but in most areas, that wouldn't be advisable.

As far as a pot, estimate the smallest pot that the roots and some bark mixture will fit into. It's hard to tell the size, but I don't think I'd go any larger than a three- to four-inch pot.