Paulo,

The choice of potting material is subject to a lot of different criteria,
but either of the options you mentioned (all spagh or a mix with some bark,
etc.) are generally suitable for Oncidiums. However, you have to be careful
to compare the results appropriately.

For example, you mention that the plant that was in all spagh seemed
generally healthier than the one in bark. How old was the medium in each
case? You say the bark was "fine". Could it have broken down? If it was
mealy, then it was probably past time to repot that Onc.

As an experiment, I took two identical Oncidium hybrids and potted one in
fresh spaghnum and the other in a mix of coco husk chips, sponge rock,
charcoal, fine Aliflor, and a little Perlite for good measure (using the
sponge rock, the Perlite was not really needed, I guess). I knew that the
spaghnum would break down long before the CHC mix. As it turned out, the
two plants performed about the same.

I generally don't use bark because I don't like how it breaks down, but that
is strictly a personal preference. Some of the most successful growers that
I know swear by bark mixes. To each his own. Keep an eye on the plants,
and if they appear to slow down in the new medium, put them back in what
made them happiest.

BTW, remember that, depending on your conditions, you may need to water a
little more now that the plants are in clay. Clay pots dry out faster than
plastic.

Have fun!

Diana