List of bands (23):

Obscura
Ocean Grove
Omerta
Omnium Gatherum
On Thorns I Lay
Onslaught
Opeth
Orbit Culture
Orbital Damage
Otherwise
Out Of Bounds
Overkill
Ozzy Osbourne
P.O.D.
Papa Roach
Paradise Lost
Parallyx
Paralydium
Parastate
Parkway Drive