Anytime I get a new orchid that is in anything other than bark I don't hesitate to repot right away. The bulb on your oncidium is rotting and I don't think it will come back and that is due to the soil. I would suggest repotting right away in either bark or moss even though it is in bloom. If you notice the blooms are fading after you repot you can cut the spike and put them in a vase so you can enjoy them without stressing out the plant.
Here is a link to an
Oncidium culture sheet, hopefully that can help answer some questions.
I saw that you posted a photo as I was typing. That certainly looks like soil, but the roots on the outside don't look so bad. When you repot rinse all the soil off and take a good look at the center of the root ball. Hopefully they look ok.