- Ordo Inferus (Death Metal-Sweden)
- Orlando Furioso (Hardcore-USA)
- Obtenebris (Melodic Death-Canada)
- Oddmongers (Thrash Metal-France)
- Old Funeral (Black Death-Norway)
- Ogronios (Folk Metal-Belgium)
- Odroerir (Folk Metal-Germany)
- Old Funereal (Black Metal-Brazil)
- Orthogenesis (Progressive Death-USA)
- Orbis (Stoner Doom-France)