The Oblyvion is a metalcore band from Mannheim, Germany, formed in 2017. They deliver aggressive metalcore with dynamic arrangements.

The Observants is a melodic death metal band from Perugia, Italy, formed in 2015. They blend death metal aggression with melodic guitar interplay and dynamic songwriting from Umbria.

The Obsidian from Albacete craft atmospheric doom metal as dark and glassy as the volcanic stone they're named after since 2018. Their sound mirrors the razor-sharp, obsidian-black depths of their Manchegan landscape.

The Ocean formed in Berlin, Germany in 2000 under the direction of guitarist Robin Staps, evolving into a large collective of rotating musicians that produced sprawling, conceptually ambitious post-metal. The band's discography traces geological and philosophical themes across concept album series, most notably the Precambrian diptych (2007) and the Phanerozoic sequence (2018–2020), as well as the acclaimed Pelagial (2013), a single continuous piece representing a descent through ocean depths. Their own label, Pelagic Records, grew out of the project and became a significant home for progressive and post-metal acts. Holocene (2023) marked a further conceptual departure.

The Ocean Collective is a progressive/atmospheric sludge/post-metal band from Berlin, Germany, active since 2000 and released through Metal Blade Records. Their ambitious concept albums — "Precambrian," "Heliocentric/Anthropocentric," "Pelagial," and the "Phanerozoic" series — map geological epochs through expansive, intellectually ambitious post-metal.

The Oddyssey from Manchester craft progressive metal under a deliberately misspelled name referencing Homer's epic since 2010. Over fifteen years of heroic compositions from the northwest has produced ambitious, journey-themed progressive metal.

The Old Dead Tree from Paris craft gothic metal that mourns the passing of ancient, withered things since their reformation in 2018. Their melancholic name drives compositions of dark beauty and emotional devastation from the French capital.

The Old Garden Geranium is a melodic death metal band from Vignola near Modena, Italy, formed in 2011. They combine death metal heaviness with melodic intricacy and an unusually pastoral band name.

The Omega Swarm is an atmospheric death/black metal project from Germany, formed in 2024. They blend atmospheric textures with the aggression of death and black metal.