The Office of Naval Intelligence is a division of the UNSC Navy that is responsible for UNSC SIGINT [signals intelligence], HUMINT [human intelligence], counter-espionage and propaganda.
Although it is under the jurisdiction of the Navy, ONI appears to report directly to HIGHCOM. It is responsible for intelligence matters across the UNSC. As such, ONI employs members of other military branches, and even civilians, to complete the Division's shadowy work.
Many of its functions and subdivisions, like the Beta-5 Division and the SPARTAN-II Program, are enigmatic, and its personnel are regarded as mysteries by mainstream naval officers. ONI personnel, especially officers, are often referred to as "spooks". Agents appear to enjoy significant latitude in the discharge of their duties, often violating moral boundaries or acquiring vast amounts of UNSC resources to achieve their ends.
The Office of Naval Intelligence often operates independently of other UNSC branches. At times it contravenes the law and UNSC protocol to complete top-secret missions. It appears to have an unusually large amount of military equipment for an intelligence service, and has a strong fleet presence. The Office of Naval Intelligence possesses a 'destroyer'-class prowler Starship, the Template:UNSCShip, which is used as a command vessel and a secure conference location. ONI also maintains the UNSC's Prowler fleet, a major source of Covenant deployment intelligence, under the auspices of the UNSC Prowler Corps. ONI also manages field intelligence with units such as ONI Recon 111 liaising with mainstream UNSC military units.
Intra-ONI dynamics are often strained with intense competition amongst its own personnel. Agents operate under heavy scrutiny with mistakes often leading to demotion or termination of employment.
The most secretive section, Section Zero's existence is not general knowledge outside ONI. Details of its role and operations are limited. Based on the records of an AI named Melissa, it is thought to be ONI Internal Affairs, charged with rooting out and eliminating illegal programs. It is very discreet in its work. The true nature and activities of Section Zero are highly classified and unknown outside the ONI command hierarchy. Zero is, essentially, an internal investigations division that polices other ONI activities. 
This section has gained notoriety as the section that keeps and maintains sealed SPARTAN-I Program file archives, including Avery Johnson's. As Internal Affairs it enforces the rules and ensures standards are met. It is authorized to use lethal force in the course of its operations.
Section One is the main branch of ONI and the branch most often utilized by the UNSC. Its responsibilities include information gathering and code breaking. Information released by Section I includes evacuation notices for colony worlds and Covenant force deployments. 
Section Two is the propaganda branch of ONI. It handles communication between colonies to reduce the spread of rumors and information that would damage morale. It was responsible for releasing details of the SPARTAN-II program to the public to boost morale throughout the UNSC. Once information about the Spartans was released to the public, Section II had to maintain the Spartans' near-mythical status within the military by listing those killed in action as MIA. It was believed that news of Spartan deaths would cripple the UNSC's already-flagging morale. Section II is also responsible for broadcasting pre-recorded human carrier signals from various locations into Slipspace to confuse Covenant triangulation technology, preventing them from discovering Earth.
Section Three is the top-secret projects division of ONI. It is responsible for the SPARTAN-II and SPARTAN-III projects, the development of the NOVA bomb and the MJOLNIR program, and many black operations. Though a few of Section III's programs are well known and are widely celebrated for their success, the Marines and top brass of the UNSC treat most with contempt. This is especially true of the Spartans, who are regarded as "freaks" by some humans. Though its overall purpose is unknown, Section III's Beta-3 Division is known to analyze captured Covenant technology such as the Kig-yar point defense gauntlet. In addition to performing black operations, Section III's Beta-5 Division operates the SPARTAN-III program, which is kept secret from the majority of Section III. This compartmentalization is to ensure secrecy and security.
UNSC Prowler CorpsEdit
- Main article: UNSC Prowler Corps
ONI Signal CorpsEdit
- Main article: ONI Signal Corps
Known ONI personnelEdit
- Template:Class - Retired
- Template:Class - Retired
- UNSC prowler
- UNSC stealth cruiser
- ONI PRO-49776
- Main article: Reach
- Main article: Onyx
- Main article: Earth
- Main article: Arcadia
- Main article: UNSC Special Forces
- In reality, the Office of Naval Intelligence is an agency of the United States Navy that was established in 1882.
- ONI's motto, "Semper Vigilans", is also the motto of the United States Civil Air Patrol. The phrase is Latin for "Always vigilant."
- ONI's pre-Halo 3: ODST insignia features the all-seeing eye, a nod to ONI's line of work.
- "Oni" is the name of one of Bungie's previous games, which predates Halo: Combat Evolved. It is also the name of an ogre-like creature from Japanese mythology.
- The ONI insignia can be used as a Firefight emblem in Halo 3: ODST.
List of appearancesEdit
- ↑ Halo: First Strike, page 49
- ↑ Halo: Fall of Reach
- ↑ Halo 3 Epsilon, campaign level, Tsavo Highway
- ↑ 4.0 4.1 I Love Bees
- ↑ 5.0 5.1 5.2 5.3 Halo Encyclopedia
- ↑ Halo: First Strike, page 98 ("ONI Section Two had done a brilliant job of preserving the fiction that Earth forces held their own against the Covenant.")
- ↑ Halo: Ghosts of Onyx, page 46
- ↑ Cole Protocol, Hypodermic-FXT-J
- ↑ Dr. Halsey's journal, January 10, 2536 entry - third page
- ↑ Halo: Ghosts of Onyx, page 326
- ↑ TVG Chats To The ILoveBees Operator
- ↑ Halo 3, campaign level, Tsavo Highway