Lay the oyster in a folded cloth on a flat surface with the flat part of the shell uppermost and hold it firmly. Force the point of an oyster knife into the hinge of the shell and carefully move the knife from side to side to prise open the shell.
Run the knife along the top of the flat shell, twisting it slightly, until you reach the muscle that attaches the oyster to the shell. Sever this and lift off the top shell.
Remove any fragments of shell from the oyster flesh, but be careful to retain the juices. The oyster will still be attached to the lower shell by the rest of the muscle. Most chefs cut through this and turn the oyster over but I prefer to leave this to the customer.