Oncidium turned yellow and shriveled up

[Carebear2]

Sorry about your loss solstice.

You can go studying and memorizing all the plant pests and diseases an orchid can pick up which some members on here like doing as a hobby but at the end of the day the orchid has died and what exactly killed it is tbh less relevant in my opinion so I don't memorize orchid diseases, I just try to avoid them with good growing practices, so I will say it was attacked by the immature baby killing fungus - the name is irrelevant although some will disagree but essentially that plant was not just "relatively young" as you describe it- it was underdeveloped, missing all leaves and far too small to survive, thus more susceptible to getting sick than a healthy plant. The roots look suspicious, I will say that estación seca has given the best advice, bulbs rot when the roots don't get enough air in wet conditions.

Bottom line is: Whoever sold that to you had just removed some underdeveloped unproductive shoots on a plant which might have survived but generally growers throw grows like these into the bin instead of trying to sell them.