Congratulations on the new baby! I think your orchid can be saved. Oncidiums are very susceptible leaf spotting fungus. I actually can't tell whether it's fugus or, like you suggested, mites or maybe both. I would use whatever you have on hand to start with. If you have an insecticide, spray all the surfaces of the leaves, psuedo bulbs and top of the medium. Wait a week and spray a second time. If you have a fungicide you can use that to start with. If you have both, I would use one for two weeks and then the other. On the other hand I don't think you want to spend money on those products just to save one orchid. You can try using plain soap and water to clean the leaves and psuedobulbs if you don't have anything else. Also it's a good idea to keep oncidiums where they'll get some good air movement.