You should be fine with the like set up of Ross. Keep the fan's running for good air circulation as well as to help dry out the medium. Oncidium's will get a fungus in the medium if left to damp for a long period of time. Been there done that and got the T-shirt
The humidity level should not be an issue if your a/c is used to resolve stuffy issues. If used throughout, a fine mist in the a.m. will be sufficient.
Basking is good, just make sure you use UV45 on those arms and legs
not to mention a dry spell will do a back wonders.