What does OCS means?

You might be talking about "ocelli". It means "one single eye". Some arthropods (insects among them) have compound eyes and ocellis, which means that have eyes that have many smaller eye-like units= compound eyes, and also ocellis = individual eye units. For instance, flies have compound eyes and Spiders have many ocellis.

Here is an article about it:

http://en.wikipedia.org/wiki/Ocelli