I obviously cannot be certain of the particular disease, but that's definitely a bacterial or fungal infection. I'd guess erwinia. If you want to save that plant, you will need a systemic treatment like Phyton 35, as a topical treatmeant like Physan (nice of them to have similar names, no?) will be ineffective. Phyton is copper-based, so may be toxic to some, more delicate plants and dendrobiums, but the oncidiums should not have an issue with it.

A good soak and then periodic sprays with Inocucor Garden Solution is another alternative, and it'll beef up your plants, too.