References to the origin of the phrase are found in two sites;
THE WORLD IS AN (ONE'S) OYSTER - "If you have a lot of money, you can have anything you want. The proverb first appears in Shakespeare's play 'The Merry Wives of Windsor' (1600).'Falstaff: I will not lend thee a penny. Pistol: Why, then, the world's mine oyster, Which I with sword will open.' Act II, Scene II." From "Random House Dictionary of Popular Proverbs and Sayings" (1996) by Gregory Y. Titelman (Random House, New York, 1996).
: : A second reference says the phrase means: "All the pleasures and opportunities of life are open to someone because he is young, rich, handsome, successful, etc. Shakespeare invented or popularized this expression." From "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997).
Take your bi-valve... er... pick...