Looks like you need to repot that guy in a smaller pot. When you do repot, clean up the dead roots and remove any squishy p-bulbs. Oncidiums should have hard p-bulbs. Do not cut off any that are still hard --even if they are a bit dark. Remove the dead spike and make sure you do not over water this guy. It looks like it either had too much love (ie water) or not enough and the roots are in severe distress. It has been surviving on the water of the p-bulbs.
Good luck and please ask if you have any questions during the process.