Players assume the roles of United Nations Space Command Marines, known as "Orbital Drop Shock Troopers" or ODSTs, during and after the events of Halo 2.

In the game's campaign mode, players explore the ruined city of New Mombasa to discover what happened to their missing teammates in the midst of an alien invasion.

Since ODSTs do not possess the Master Chief's advanced armor and reflexes, they cannot jump as high, move as fast, or survive large falls.

[5] The player's head-up display (HUD) includes a "VISR" mode that outlines enemies in red, allies in green, and items of interest in either blue or yellow.

After finding a piece of evidence left behind, such as a sniper rifle hanging from a power line, the player will enter a flashback mission, where they assume the role of the missing soldier six hours earlier.

The game begins with Dutch (Adam Baldwin), Romeo (Nolan North), and Mickey (Alan Tudyk) discussing the Covenant Supercarrier above New Mombasa, with the Rookie asleep nearby.

If the campaign is completed on the Legendary difficulty level, a scene shows the Prophet of Truth overseeing the excavation of the Forerunner artifact buried beneath the Superintendent's data core.

To increase the depth of the surroundings, Bungie added touches such as advertisements, trash collectors, and civilian versions of previously seen military hardware.

[34] Though the game engine remained unchanged, graphical enhancements such as fullscreen shader system and parallax mapping added greater detail and realism.

Designer Lars Bakken described one advantage of the game mode as providing a more friendly multiplayer environment than hardcore players online.

Fourth Wall Studios was formed by former members of 42 Entertainment, who produced the highly successful I Love Bees alternate reality game to promote Halo 2.

[39] Members of Bungie were fans of the television series Firefly, and in Halo 3's development brought in several of the actors to fill Marine voice roles.

Staten explained that it was too early in the game's development to spend a large amount of time on the trailer, and it would not have allowed Bungie to add touches for fans to discuss.

[62] By April 2009, market research firm OTX reported that ODST was the most highly anticipated video game,[63] a spot it continued to hold in late August.

[69] A live-action trailer titled The Life was posted on the Internet in September and later used in television spots as part of the promotional material released for the game.

[70][71] Filmed at several locations in Budapest, Hungary, The Life follows an ODST named Tarkov from a military funeral where he is inspired to become a soldier, through training and combat and up to the moment he becomes a leader in battle.

[73] Bungie supplied 3D geometry that allowed the company to quickly create accurate representations of in-game items, and provided input on the UNSC dress uniforms.

Bungie and Microsoft sponsored an official launch event at the Experience Music Project and Science Fiction Museum and Hall of Fame in Seattle, Washington.

[80] ODST took the top spot on Australian game charts on release and, after being outsold by FIFA 10 in early October, reclaimed the best seller position.

[88] Expecting sales of the game to increase as players wanted to access the Reach beta, UK retailers slashed its price in April 2010.

[93] Time's Lev Grossman wrote ODST was a "milestone" as it proved "Bungie can use the same instrument to play in totally different key," adding to the longevity of the Halo franchise.

[110] 1UP.com's Jeremy Parish noted that while ODST had its flaws, its greatest success was bringing together players who preferred either the multiplayer or campaign portions with a game that would satisfy both camps.

[103] Those who disagreed included Ars Technica's Ben Kuchera,[111] GamesRadar's Charlie Barratt,[102] Eurogamer's Tom Bramwell,[97] and IGN Australia's Narayan Pattison.

[6] Technology company Digital Foundry said that, despite welcome improvements in AI, the main flaws from Halo 3's graphics engine—namely a sub-720p native resolution and lower-quality human faces—remained.

[115] McCaffrey wrote that while the music of Halo 3 was "too familiar", ODST had freed O'Donnell to "craft his best work yet", which the critic considered good enough for a standalone purchase.

[106] A significant departure from the consensus was offered by Kuchera, who wrote that the saxophone touches "[sound] like the softcore porn they show on Cinemax after midnight",[111] and GameSpy's Anthony Gallegos, who felt that the music did not mesh when extended into combat segments.

[99] The staff of Official Xbox Magazine UK wrote that the gametype's depth and options made Gears of War's comparable mode look "simple" in comparison, and that Firefight extended the life of the game.

[107] In contrast, Bramwell felt that in Firefight "there is both a sense of futility in the knowledge that death is only a matter of time and odds, and fatigue in the realisation that many levels play out just as they did in the campaign, except a bit more so".

[117] Halo manager 343 Industries director Frank O'Connor said that he was glad ODST "didn't take off", in his view, because it allowed people to focus on Reach and its launch and marketing.

[120] Chris Kohler from Wired ranked ODST as the third best Xbox 360 title of the year, writing that the game "injected a fresh breath into the staid corridors of Bungie's shooter franchise".