This is basically the only Oncidium orchid that has this issue. And I have plenty of them.

It's recieving a few minutes of direct sunlight a day through a small slit above, and the rest of the day it only gets indirect light.
That is the way it's recommended, since they supposedly need lots of sunlight, way more than other orchids like Phalaenopsis.

Others have a way brighter spot and don't look that way.

Might it be a bacterial infection, or a virus?

I've also looked for pests, but didn't find any.

It's also a pretty new specimen in my collection, is that due to transplant stress?

I don't know what is the problem - can you give me your guesses?