what you would be doing now is called an 'emergency repot' DO NOT BE ALARMED! its not a real emergency, but the stores usually dont care for the orchids correctly if its not an specialty orchid nursery. so the onc. has probably been in that medium for a while. and with bad watering habits at the store, its best to get it into some fresh medium.
i dont have any oncidiums, so i dont know about the potting MEDIUM. but i do know that ocidiums are epiphytic orchids. that means that they live on trees in nature. so they like good aeration in the potting medium. i think most people either grow them on mounts or in a coarse bark mix. something like this
Oncidium and Seedling Classic Orchid Mix hopefully someone will chime in with more information on the temperature, llight, and watering needs.
ps. correct me if i got anything wrong