Usual Oncidium care - moderate light, media lightly moist.
Silje - remember that pebbles are much smoother than lava rock and won't retain moisture. Maybe try bark without the moss added?
I haven't had much trouble with overwatering most Oncidium alliance (except Onc Twinkle)
but have overwatered a great many Phals!
Bamboo skewers or wood chopsticks in the media are a great help to monitor moisture in the media.
I now usually pot my Onc alliance in coconut husk chips mix, tho they did well in small bark mix also, and even in a pinch the mix I make for Cyms and also use for Paphs (mix of medium and fine bark, perlite, pumice). But they do seem to really love the coconut husk chips (retains moisture well, yet still very airy, and drains well - tho I have seen people posting here having trouble with it as a growing medium).
Also, you can add perlite, pumice, charcoal (horticultural) to your mix to aid drainage, keep mix from compressing. Another thing I do is to put a piece or two of styro packing pnuts in the root mass before repotting to create an air pocket - helps keep air to the roots, and media to dry more evenly.
Good luck!