Oh boy - you've put me on the spot! As I indicated this plant was given to me - a rescuee. It was originally in sphagnum, it's owner decided it needed to be repotted & treated it like a plant that grows in soil. So the sphagnum remained intact, it was up-potted (from probably 4") to a 10" pot! :sob: The potting media was gravel on the bottom couple of inches & bark filling in the rest! I can guarantee it was not happy, time to experiment & find out what will make it happy. I don't feel I'm a very good oncidium grower as they do well or croak with me! It somewhat decided to divide itself, so I now have 3 plants, all potted differently - 1 in a bark mix (& this is the one blooming), 1 in S/H. These 2 plants are grown on a lower bench in the GH getting morning sun. The 3rd plant I replanted in fresh sphagnum & put it in a vanda type basket & it's hanging off the ceiling in the GH getting more sun than the other 2 but still partially shaded from later afternoon sun. All plants have produced new growths/bulbs & none, so far, have the black spotting/peppering on the new leaves that one sees so often with oncidiums, so I'm guessing that light & water is correct.
So the short answer is VARIABLE!