A couple of questions...

[King_of_orchid_growing:)]

The calcium present in the oyster shells need to be crushed into a fine powder and made water soluble (just as Roberta mentioned) for it to make any kind of a difference or be reasonably bioavailable in a significant way for the plants, particularly with epiphytic orchids or lithophytic orchids. Any calcium that is weathered out of whole oyster shells are quite minimal and must be done over a long time and under the right chemical conditions. Sure, plants have mechanisms to chemically change the environment they grow in, but it has limitations, so you can't count on this for the oyster shells to be of good use.