Good question! Maybe add sphagnum to the top of the pot until it's large enough to divide or the old pseudobulbs die off??? Hopefully some experienced growers will chime in.

It is tricky and I will offer how I deal with this dilemma. I don't know whether it is correct, however it works for me.

My main concern is the first, largest bulb. I always wait to repot when new growth is beginning on the first bulb. I place the large bulb with its new growth in the potting mix. I do not worry too much about the old bulbs. They offer support to the large bulb but will eventually die off.

The smaller back bulbs get buried as necessary to make sure the first bulb is potted at the proper depth.

I sometimes use sphag in that part of the pot where the buried smaller bulbs will go.

I use small bark mixed with sphag for Oncidiums but might use a top dressing of sphag in the place where the back bulbs will get buried. Beneath the sphag is the same bark/sphag mix. The top dressing of sphag will dry out quickly and prevent the older bulbs from rotting.

Oncidium growth is like a ladder. The newest bulb wants to climb higher. Just support those back bulbs until they die off. 'Luvmyorchids' stated exactly what I do.

Remember to place those back bulbs along the back wall of your pot to leave good growing space for the newer growths.

Jane