Black spots on Oncidiums
 

Can you tell from the attached photo what's eating my Oncidiums?

It seems that no one has replied to your post, so I'm going to offer the bit of information that I have on oncidiums. The word is that oncidiums suffer from dark spots which do not cause much harm, but whether or not the spots on your plant are the type that is referred too, it's hard to tell. If no one more knowledgeable replies to your question, you might try to do some research on this condition.

One theory I have heard is that of the buildup of salts exuded through the leaf stomata, resulting in some localized toxicity.

I have no idea if that has any validity, but I have noticed over the years that it is 1) common, 2) only an aesthetic issue, as the plants don't seem to "mind" it, and 3) reduced in occurrence when the RH % is kept high.