[fred]

just plain household lighting is not good enough
I have several Oncids and it is normal to loose the odd leaf
Brookn has given you the culture sheet for Oncids please give that a read

I am not saying that you have to do this
if this was my oncid I would be potting this into sphagnum moss
having said that one has to know how to use sphag as a medium a fine bark mix would also be good

I would not soak the plant for any longer than half an hour if it was in a bark mix a light mist on the leaves with a pressure sprayer would also be a good idea

the other thing is don't panic and try to have some patience and follow the advice given to you