Tap the shells of any mussels that gape; if they don't close, discard. Place mussels in a 5- to 6-quart pan. Add 1/2 cup water, cover, and bring to a boil over high heat. Reduce heat and simmer until shells pop open, 5 to 10 minutes. Pour juices from pan and save.
In a 14-inch wok, 14-inch frying pan, or 5- to 6-quart pan over high heat, melt butter with garlic and pepper and stir until garlic is golden, 2 to 3 minutes. Add oyster sauce and reserved mussel-cooking liquid. Boil over high heat, stirring often, until sauce is reduced to about 1 cup, 5 to 6 minutes.
Pour mussels into wok; stir over high heat until well coated with sauce, 3 to 4 minutes. Ladle mussels and sauce into bowls; pluck mussels from shells and sop up sauce with bread, if desired.