ONI Alpha Site

ONI Alpha Site is one of the flashback levels in Halo 3: ODST. It is set in the ONI Alpha Base in New Mombasa.

The level is set two hours after the Squad's drop into the city, and four hours before the Rookie's awakening, from Dutch's point of view. He and Mickey are defending the ONI facility with the help of the city's Superintendent, NMPD, and Marine forces.

The level opens as they are setting demolition charges to destroy the bridge leading to the complex to prevent the Covenant forces from accessing it. After the charges are armed, an NMPD officer commands the Superintendent to unlock a keypad on a bridge toll booth so they can remotely detonate the charges. The AI is hesitant at first, but eventually lets Dutch destroy the bridge. After this, they fall back deeper into the facility, fighting off more waves of Covenant until they are forced to retreat into the ONI building itself. Dutch and the NMPD officers fight off the squads of Brutes, Grunts and Jackals that continue to push into the building. Eventually the fight is brought up to the roof of the facility, where the final Covenant infiltrators are killed. A Pelican picks up Dutch and Mickey, and the level ends as Mickey remote detonates the charges in the ONI facility, destroying it.

Trivia

 * During the drop sequence when the player is in a SOEIV the ONI Alpha Site and its surroundings can be seen, including the bridge that is destroyed two hours later in this mission.
 * If playing on Co-op with campaign scoring active, the host will recieve points for any Covenant who are killed by the bridge's detonation, often resulting in large multi-kills such as Killpocalypse or Killionaire.
 * Inside the base is a memorial wall with hundreds of names of the people who have died in the war. Dr. Catherine Halsey and Adm. Preston Cole are listed, along with some humorous names, like London Bridges.
 * At the part where you have to defend until the elevater comes, if you jump over the walls (where the elevator comes up) and fall to the bottom then go into theater and go to the bottom you will see an un charge explosive, this is either a glitch or an easter egg or it was placed there on purpose, it is still odd for it to be the only one in the shaft. It is also on the firefight map Alpha Site, but unarmed(green, red armed).