noun, often attributive \’ȯis-tər\

Definition of OYSTER (Merriam-Webster)

1a : any of various marine bivalve mollusks (family Ostreidae) that have a rough irregular shell closed by a single adductor muscle and include commercially important shellfish b : any of various mollusks resembling or related to the oysters

2: something that is or can be readily made to serve one’s personal ends <the world was her oyster>

From Wikipedia:

 William Shakespeare (c:1600)  is credited with first using the phrase in his play “The Merry Wives of Windsor” in Act 2, Scene 2, 2-5

Pistol: Why then the world’s mine oyster/which I with sword will open.