Selling you a plant with aphids was not a good thing! They should refund your money in my opinion. I would pot it up in a bark charcoal mixture such as Schultz. I would water it as needed-(when approaching dry. I think you need to question this seller's advice...
Here are a couple of threads to click on.
AOS | Oncidium http://www.orchidgeeks.com/forum/new...f-orchids.html
Good luck!
