Hi
Tim gave you a good advice, but I want to stress some points
I don't have these Oncs, but have anothers similar in culture. Don't know your weather, so you have to adapt the tip to your setup.
The best advice someone can give you (my opinion, of course) is that they like frequent watering when in grow but with excellent drainage (As Tim said).
You
must be sure that they dry fast, ensure a wet/dry cycle of 2-3 days. In my setup I do that by mounting them or with the use of very coarse pieces of bark.
They also need a dry season in winter.