With your Oncidiums, yes I would. Just keep those finer roots in mind. Imagine a small particle size in your media. The roots easily come in contact many times with a fine mix.
But with a coarse mix, the roots also come in contact with the media BUT with the coarse mix, more air moves through. More air makes it dry out much quicker then a fine mix! It is the very rapid drying that is not good for Oncidiums. The more time that they spend in contact with moisture, the more those roots absorb the water.
I have never used coir. Therefore I do not have experience with it in order to be able to recommend it.
