Californian Oyster scale or simply oyster scale is an absolute shocker of an orchid pest.
My advise on its control is so simple. Destroy the orchid by disposing in the bin.
It is far too hard to control and remove. It is nearly invisable and hides down under the compost on the underside of the rhyzome
It is usually introduced in to your orchid collection by an old second hand infected plant.
Neglected, tired, dried out orchids seem to be far more prone to Oyster scale.
It is spread by its own means slowly, but is spread much faster by ants farming and moving the scale around your collection. Yes they actually farm scale for its honeydew secretions.
Should the orchid be highly valuable, emmotionally or monetry, then confidor pills and surgically removing the infected canes may work. A drench of Diazonon is also reccomended. AS you can see on the rolled leaf photos below, these scale seem to know how to hide so well.
1 mill per 1 litre of diazonon
Applied as drench on badly effected
Or spray if lightly effected