How I would describe ONI:
The Office of Naval Intelligence is a division of the UNSC Navy that is responsible for 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 theSPARTAN-III 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.
Here for more detail