What I use successfully for strap-leaved Oncidiums (like O. sphacelatum) and intergenerics is the coarse and medium woody material that I screen out of municipal compost (I do this before using the screened compost in outdoor ornamental planters).
The coarse stuff is woody chips, similar to medium to coarse orchid bark. The medium stuff is somewhat smaller. Both do drain well, but retain a little moisture. The material has a little nutrient-rich compost adhering. It has been composted, but only for a month or two, so not broken down too much. My Oncidiums seem to love the stuff.
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