Halo 3: ODST takes place during the events of Halo 2 , starting right after the mission “Metropolis” in New Mombasa, and it overlaps with the early part of Halo 3 for some of its later scenes.

Timeline

  • The opening of ODST lines up with the Covenant ship’s slipspace jump at the end of Halo 2’s New Mombasa sequence.
  • Most of the campaign happens over the following hours as the Rookie and the other ODSTs search the city.
  • The broader story finishes before the main events of Halo 3 fully play out.

Simple way to place it

Play it as:

  1. Halo 2
  2. Halo 3: ODST
  3. Halo 3

Quick answer

So, the shortest version is: ODST happens during Halo 2, not after it, and it bridges into the opening of Halo 3.