wrinkled pseudobulbs, spots behind the leaves of oncidium plant
 

Hi,
I have brought an oncidium plant home about two weeks ago. It was fine at first. But within 9 days, the flowers started to drop off and all the pseudobulbs got more wrinkled. Soon, spots have started to appear under the leaves and some spots are even surfacing on the front side of leaves. The leaves also look kinda limp and lifeless. A stem had yellowed as well. The overall condition is really scaring me. Is it dying? Does it need more water or some fertilizer? Please help.......

Just inspect the media within the depths of that container. Is the media (and the roots embedded within it) very wet? Or is it very dry?

If it's very dry most of the time, then it's possible that the orchid is not getting enough water into the roots.

If it is very wet - and if the roots aren't adapted to handle that sort of mostly wet conditions, then the roots can begin to die ----- which can also lead to roots not functioning properly (such as not able to get water into the plant) ---- so also leading to drying out of the plant.

With many orchids - just need to be careful with over-compensating ------ as in ..... if the conditions are currently very dry ....... then sure ----- more watering might be needed, but got to stay in control of the situation ---- since making the media become too wet for long periods of time can then make the plant's health take a dive as well. So it's mainly about keeping situations under control.

First, Welcome!

Dropping of flowers is normal. If you got it in bloom, you don't now how long they were open, but after 4-6 weeks from opening, they're done. Your care will help it bloom again (Some bloom twice a year, some just once, so be patient)
I agree with SouthPark that it is very likely wanting more water. Oncidiums don't want to dry out. If you see new growth starting, you can repot it... If it were my plant, I would use small bark, or sphagnum. But the coconut chunks that appear to be its medium should work fine, if that is the medium that is available to you. Whatever the medium, you want it to drain well (air in the root zone is very important, and if the existing medium is old then it will tend to become airless. But for sure, it doesn't want to ever dry out.

I agree the plant looks as though it has not getting enough water for some time. Pseudobulbs on these are normall without wrinkles when receiving adequate water. Some of the leaves have horizontal wrinkles or pleats. This indicates insufficient water when the leaf was growing.

In the photo with filename ending in 223, showing leaves, I think I see evidence of spider mite attack. These are very tiny spider-like creatures that weave fine webs under leaves, and suck juices from plants. They are commoner on thin-leafed orchids like Oncidiums.

Take a piece of tissue paper and press it against the underside of the leaf. Pull it toward the tip, holding pressure on the tissue. Do you see brown or reddish streaks or stains? If so, you have spider mites. Let us know and we can tell you more.

It looks to have a very well ventilated pot and coarse media: you may need to look at watering daily or every other day. If the media is staying wet/moist already, it may have a bad root system, but it seems to me that it would be really easy to underwater in its current set up.