There are two epic enemies on Ossus. Neither boss has a codex entry in the epic enemies section, but both are required for achievements.
The Kil'Cik the Swarm Lord world boss.
Codex type: Achievement: Epic Enemies
Codex location: Ossus: Ossus Canyons
Codex coordinates: 1325, 520
Faction: Both
Class: Bounty Hunter - Imperial Agent - Jedi Consular - Jedi Knight - Sith Inquisitor - Sith Warrior - Smuggler - Trooper
Associated Achievement: Advanced: Blood and Oil | Blood and Oil | Elite: Blood and Oil | Sick Kill
The R8-X8 world boss.
Codex type: Achievement: Epic Enemies
Codex location: Ossus: Ancient Ruins
Codex coordinates: 946, -730
Faction: Both
Class: Bounty Hunter - Imperial Agent - Jedi Consular - Jedi Knight - Sith Inquisitor - Sith Warrior - Smuggler - Trooper
Associated Achievement: A Date with R8-X8 | Advanced: Blood and Oil | Blood and Oil | Elite: Blood and Oil