I feel your pain. I've been growing orchids for ten years and I've always had trouble with oncidiums. I mounted one Onc. Sweet Sugar on a piece of tree fern fiber about eight months ago. I didn't use any sphagnum but I'm sure it can be done that way. It's been growing very well and put out six new growths. The only problem is that oncidiums require quite a bit of moisture so I wind up watering mine almost every day. Good luck.