- “Eye of the Prophets”— Translated Covenant Definition of "Ossoona"
- Main article: Rank
Unlike most other Covenant rankings, "Ossoona" was a temporary rank bestowed by the Prophets of the Covenant upon specially selected Sangheili warriors, making it comparable to the typical rank of the Arbiter. Ossoona are roughly equivalent to expendable spies or assassins, sent on high-risk missions.
Due to a lack of intelligence on the United Nations Space Command, the Prophets had begun to pull a handful of Sangheili from active duty into temporary service as Ossoonas. In order to be chosen, these individuals had to have the rank of Major. This is because the higher ranking Sangheili are "worth" more in active duty, whereas the lower ranking Minors were considered too inexperienced and liable to either become easily compromised or abandon their mission in pursuit of their own personal glory.
The mission of an Ossoona was to collect information on the enemy while avoiding any type of combat. Many of those Sangheili felt that there was little honor in such an assignment, but they would do as commanded due to their reverence for the Prophets. Covenant leaders felt that zeal was a quality to be admired, but an espionage job would help tame young Sangheili and teach them the finer arts of stealth in battle.
The title of Ossoona was usually bestowed for a single mission, though a series of missions were sometimes given. An Ossoona would always be equipped with Active camouflage generators and devices similar to UNSC Helmet Recorders. They were usually equipped with a Plasma Pistol and Plasma Grenades as well as energy shielding just in case they were to be discovered. Ossoona missions tended to include non-combat intelligence gathering, such as capturing high ranking enemy personnel, equipment, AIs, weapons, etc.
- Record footage of the Human ship on his helmet recorder.
- Capture the Human Artificial intelligence.
- Capture senior personnel.
Because many Sangheili were reckless and tended to destroy everything in their path, the Prophets (Hoping to learn more about the Humans, including the location of Earth) sent 'Nosolee on a special mission to gather intelligence. Even though he was killed when Captain Keyes shot him in the head with a M6D Pistol, the attendance of the Ossoona in the life pod Captain Keyes had fled aboard alerted the Covenant to his importance, and ensured that they captured him instead of killing him along with his party.
- The Aztec-Tlaltelolcan Ossoona Codex, a writing system that to this day cannot be translated, likely inspired the name of this rank.
- Stealth Sangheili differ from Ossoonas in the fact that they are used for combat/confrontations with the enemy. Ossoonas are designed to gather information about the enemy while avoiding coming in contact with possible enemies.
- Though Stealth Sangheili have their Jiralhanae counterparts, Stalkers, there is presently no known Jiralhanae "Ossoona". This may be because the Covenant had already captured enough data on the Humans, or simply because Jiralhanae are simply unable of completing stealth missions successfully. This may be due to their lack of self-control and their body structure.
- It is shown in Halo: The Flood that many Sangheili prefer to be fighting with their comrades, rather than be an Ossoona and sneak around, something that they see as a cowardly act.
- It is possible that the Spec Ops Sangheili seen at the beginning of Winter Contingency is an Ossoona, since it is carrying a data pad, which matches the role of an Ossoona of intelligence gathering, flees when it sees NOBLE Team, however it is carrying a plasma repeater (which previously stated should not be used by them), and is cloaked. It also would be likely that the Covenant would send an Ossoona to gather information about UNSC defenses.
- Halo: The Flood (First appearance)