There are 3 locations codex entry on Ossus. These codex entries are not required for any achievements, and two are faction restricted.
The Strike Base XR-484 codex entry datapad.
Codex type: Locations
Codex location: Ossus: Imperial Command Area
Codex coordinates: 5508, -4560
Faction: Empire
Class: Bounty Hunter - Imperial Agent - Sith Inquisitor - Sith Warrior
Associated Achievement: None
The Great Jedi Library of Ossus codex entry datapad.
Codex type: Locations
Codex location: Ossus: Jedi Library Ground: Jedi Library Ruins
Codex coordinates: -671, -198
Faction: Both
Class: Bounty Hunter - Imperial Agent - Jedi Consular - Jedi Knight - Sith Inquisitor - Sith Warrior - Smuggler - Trooper
Associated Achievement: None
The Hidden Jedi Colony of Ossus codex entry datapad.
Codex type: Locations
Codex location: Ossus: Jedi Colony
Codex coordinates: -228, 2633
Faction: Republic
Class: Jedi Consular - Jedi Knight - Smuggler - Trooper
Associated Achievement: None