There's a lot of things the black spots could be, from insect damage, disease, or virus. However, I don't think you need to worry too much, because Oncidiums are also known to get black spots from high sun exposure, which I think is the most likely cause. They are a little unsightly, and if they bother you, situate the plant so it gets less light, but still enough to bloom. The spots won't go away, but new ones won't form. If the plant is not receiving excessive light, and new spots form, then you can worry about what might be causing it, but for now, I would take a wait and see approach. Other than that, the plant looks very healthy.