Please note, I don't know...and these are wild guesses. However, my intuition is saying something about the roots. It may be too much water, or it may be a salt buildup. Take that with lots of grains of salt and lets see what others think.

I'd lean toward too wet as well. Perhaps to the point of losing roots. I'd take a look at those roots.

tj

This is the same plant I have to repot. I made a separate post where you can see the whole thing.....I guess I should repot sooner then later. I was gonna wait another month or so when I put them outside.

Okay...so I just wanted another opinion, but I'm going to say it's over watering. They loved it at first because I wasn't keeping them moist enough. I guess I've been a little heavy handed. Salt build up I doubt because I don't fertilize for 8 months out of the year and when I water them they drain straight through....Salts would have washed out by now.

This maybe helpful http://www.cloudsorchids.com/doctor/dspots.htm

So strange. I haven't fertilized in 7 months and these spots started developing two or three months ago.

Could your water quality have changed over winter? the total disolved solids in your water (assuming you are using a municipal source) can change with the seasons. The total disolved solids in your water could be too high. Have you ever checked it?

Also have you checked the roots for damage? This may give you some clarity on whether your issue is over watering or burning.

You can sometimes tell if you are over watering because there is a brown discoloration at the base of the pseudobulb. This is an indication of the onset of root rot.

Maybe to be safe I should start watering them with water from my zero water pitcher....that would eliminate that variable....for now. Thing is, when they are outside, like they will be in another month, all will be well. Plenty of bright indirect light, plenty of breezes, the plant will be firing on all cylinders and the problems will go away. It's when things slow down that the issues arise.

I also water with municipal water all summer and don't have any problems...

Oncids aren't supposed to dry out, right?

I like mine to approach dryness in the winter but if they dry completely their pbulbs wrinkle. In the summer I grow them damp. The amount of water needed in the summer vs the winter is different for most orchids, including your oncidium. Plus in the summer outside the are movemnt is probably. Things dry out more quickly including the potting mix and the leaves. This decreases the likelihood of fungal infections. You can trim the spotted areas and treat the plant and back off of watering until they go outside.

Check out the roots and the base of the pseudobulb.