Omens of Plague is a Buenos Aires metalcore/melodic death metal band formed in 2009, featuring vocalist Gonzalo Roland, guitarists Francisco Cañardo and Nahuel Hadouken, bassist Rodrigo Ferreiro, and drummer Pazuzu Imdugud. The band released their debut full-length The Merciless Beyond on May 10, 2017, a 12-track record that drew praise for its technical guitar work and melodic death metal songwriting.

Omni Gear emerged in 2016 from the ashes of Rebel Machine, a heavy metal act the same members had been running since 2005, bringing a harder power metal focus to the new project. Their lyrical universe runs through science fiction, political commentary, and social themes, and the band placed the track "Fever Dream" on the Natal Metal Compilation III released through Dio Records.

Omniscience play an expansive, keyboard-laced atmospheric black metal anchored by multi-layered guitar work and the dual vocal interplay of Kalash and Archon. Their 2021 album Ancient Spell, released through Careless Records, draws its lyrical imagery directly from Magic: The Gathering lore, with tracks like "The Coming of Iona" and "Gates of the Mindless Temple" treating the game's mythology as genuine mythological source material.

One of Them is a Porto Alegre thrash metal outfit whose 2019 album Blind Faith, recorded at Hurricane Studio with producer Sebastian Carsin, brought together vocalist GG Mussi, guitarists Jeff Witt and Ivan Santos, bassist Alexandre Guterres, and drummer Jonas Koehler. The record marked a new phase for the band, arriving roughly a decade after their debut EP I Am One of Them and featuring a Portuguese-language track for the first time.

One Third of the Angels is a black metal duo formed by Ricardo Echeverria, also known from One Cross, and Wagner Santos, who plays in Nox Caelum and Revogar. The project surfaced in 2023 with the single "My Darkest Memory," channeling a cold, atmospheric approach rooted in the South Brazilian underground scene both members have long inhabited.

One Thousand Dead play a direct, old-school-rooted blend of thrash and death metal in the tradition of early Brazilian extreme metal. Their debut full-length Guerra, released through RTR Records in November 2022, compiles ten tracks that the band began developing during the COVID-19 pandemic after signing a multi-album deal with the label.

Onihime is a one-person brutal death/slam project based in Corrientes, Argentina, blending extreme metal with anime-themed imagery and horror. The project has released two albums, Waifu of Madness and Kult of Lamashtu, operating under the full moniker Onihime Death Cult.

Oniria is a gothic metal project from Buenos Aires active since 2005, currently operating as a solo endeavor by multi-instrumentalist and vocalist Guillermo Manolio. The band began with a fuller lineup that included guitarists, bass, and keyboards before consolidating around Manolio as the sole creative force, releasing a self-titled demo in 2005.

A one-man old school death metal project helmed by Cláudio Slayer, Open the Coffin worships the HM-2 chainsaw tone and traffics in horror imagery centered on zombies, cemetery rituals, and slasher lore. Their 2019 debut EP "Only Death Prevails" was tracked at Black Hole Studio in Natal and released through Black Hole Productions, delivering a raw, suffocating sound indebted to the Swedish death metal tradition.