Clean oysters vigorously under cold water with a scrub brush. Grasp an oyster in the towel, with the flat-side up and the hinge (where the shells meet) exposed. Poke the knife into the hinge. Twist the knife back and forth—like turning a key in a lock—until the top (flat) shell pops open.
Slip the blade along the inside of the top shell, severing the muscle (the oyster will rest in the bottom shell) and being careful not to spill the liquid. Discard the top shell.
Run the blade under the oyster to cut the attached muscle. Wipe the knife clean; repeat. Serve the oysters on a bed of crushed ice.