O'Donnell collaborated with Michael Salvatori for many of the scores; he has also directed voice talent and sound design for the Halo trilogy.

O'Donnell moved to composing video game music when his company, TotalAudio, did the sound design for the 1997 title Riven.

O'Donnell describes his upbringing as "typical"; he received piano lessons and wanted to start a rock band when he was in junior high school.

O'Donnell talked to his friend Michael Salvatori, who had his own recording studio, and offered to split the profits from the job with him; the two became constant partners.

[4] After completing a film score and a few commercials, the two decided to quit their day jobs and produce music in Chicago;[4] they founded a production company, TotalAudio.

[5] After fifteen years of composing for TV and radio commercials, he decided that he wanted to work on game soundtracks and move on from commercial-sounding music.

[6] O'Donnell became acquainted with the game's developers, including brothers Rand and Robyn Miller, and was hired four years later[4] as a sound designer for Myst's sequel, Riven.

[7] Among the games Riven's developers would play in their downtime was a title called Marathon, created by Chicago-based Bungie.

The company later composed the music for Valkyrie Studio's Septerra Core: Legacy of the Creator, during which O'Donnell met Steve Downes, whom he would later recommend as the voice actor for the Master Chief.

After talking with Joseph Staten, O'Donnell decided the music needed to be "big, exciting, and unusual with a classical orchestra touch to give it some weight and stature.

In an interview, O'Donnell stated that he has always approached music from the keyboard, and that at the Electronic Entertainment Expo—where the trailer would first be shown—he had a feeling that, "no [other announcement] would start with a piano.

[4] Due to ODST's shift to a new protagonist, O'Donnell created music that was evocative of past Halo but branched in a different direction.

[18] Since Bungie was aiming for a smaller, detective story feel, O'Donnell felt that a jazz-influenced approach worked best in echoing the film noir atmosphere.

[21][22] In 2015, music from the Halo series was voted by listeners into the Classic FM Hall of Fame for the first time, reaching position 244.

[23] Early in the video game Destiny's development, O'Donnell began composition of an eight-movement symphonic suite entitled Music of the Spheres.

[35] The Halo 3 Original Soundtrack was released in November 2007, and featured a fan contribution that was the select winner from a pool of entries judged by O'Donnell, Rodgers, and others.

[12] O'Donnell prefers to write music towards the end of the development cycle, because he would rather score the final timing for things like cinematics and gameplay changes.

O'Donnell pushed Bungie to spend money on hiring singers and musicians to record the theme before Macworld as a way to present a strong showing.

Instead classical music by Beethoven, Brahms, and Barber and progressive rock groups like Jethro Tull and Genesis informed O'Donnell's taste and works.

[38][39][40] In March 2024, O'Donnell announced on his Discord server his intention to run as a Republican Party candidate in Nevada's 3rd congressional district in the 2024 House of Representatives elections, challenging incumbent Democrat Susie Lee.

[43] The Nevada Independent reported that O'Donnell has a net worth of nearly $74 million, much of which is held in stock holdings through individual retirement accounts and investment portfolios.