ah, okay, "the world is your oyster". That's a common expression. Oyster is a noun here, though. Basically it means "there are tons of opportunities out there just waiting for you" I think this expression exists because oysters can have pearls in them, but you can't always see them without going out there and opening them. The world is sort of like that too: it's full of opportunities and resources just waiting for you to go out and take advantage of them.

I can't say for sure without context, but probably "containing oyster" or "related to oysters" An oyster fork, for example, is a fork used to eat oysters and other seafood. Oyster sauce is a sauce made with oyster. If you tell me what context you saw oyster being used as an adjective, I'd be happy to provide a more specific answer.

Can you give us the context?