My headcanon has always been that co op takes place in an alternate reality where Noble 6 got on the Pelican with Keyes : r/halo
Firefight (Halo 3: ODST) - Halopedia, the Halo wiki
Halo MCC April 11 Update Deployed for Cross-Platform Co-Op for Halo 3 & More
Co-Optimus - News - Halo 3: ODST Designed for Co-Op Since Day 1
Halo 3: ODST's Co-op Firefight Mode is Coming to The Master Chief Collection This Summer
Co-Optimus - Review - Halo 3: ODST Co-Op Review
Halo 3: ODST - multiplayer hands-on | GamesRadar+
4-Player Co-op Glitch | Halo Alpha | Fandom
Halo on X: "Flood Firefight. Halo 3 and ODST co-op crossplay. Full custom game browser support. And, that's just for starters. The latest update for Halo: The Master Chief Collection has officially
Co-Optimus - Video - Halo 3: ODST Video Blowout
Halo 3: ODST - Wikipedia
Playable hunters in halo 3 odst! coop is possible on the original odst build.