With it, the band decided to create "a classic Dream Theater album",[1] drawing upon their various stylistic influences while trying to make the music less complex.

Dream Theater promoted the album on a year-long world tour, with the majority of concerts lasting almost three hours and featuring a different setlist each night.

The tour finished at Radio City Music Hall accompanied by an orchestra; this performance was recorded and released as a live album and concert video entitled Score.

After completing a North American tour supporting one of their main influences, Yes, in summer 2004, Dream Theater took a two-month break.

[8] The band reconvened at The Hit Factory in New York City in November 2004 to begin work on their eighth studio album.

The Hit Factory, a studio in which artists such as Michael Jackson, Madonna, Stevie Wonder, U2 and John Lennon had recorded, was earmarked for closure.

[1] Keyboardist Jordan Rudess described it as "really going back to creating a real band effort, as well as drawing upon all our various stylistic influences.

Drummer Mike Portnoy dismissed claims that Octavarium was an attempt to write a more commercial album, stating that the band simply "[has] that side to [them].

The orchestra was selected based on their sight reading ability, allowing all their parts to be recorded in a maximum of two takes, even though they had never seen or played the music before.

[11] Octavarium begins "The Root of All Evil" with the final note of the band's previous album, Train of Thought with "In the Name of God".

"The Root of All Evil" is the third part of Portnoy's Twelve-step Suite, a set of songs from various Dream Theater albums which describe his journey through Alcoholics Anonymous.

The instrumental introduction, heavily influenced by Pink Floyd's "Shine On You Crazy Diamond", was performed by Rudess using a lap steel guitar and Continuum.

[14] The song ends with the introductory theme of "The Root of All Evil" before fading out (instead there is an alternative version for some releases, with the reprise flute from the first part of "Octavarium" playing in the background).

[16] Portnoy stated that "[he] chose to do this mainly [emphasis removed] to build anticipation for the big 'official' unveiling next week", although noted that "the repeated requests to refrain from spoilers and links for the new album against the band's wishes were frustrating".

Writing for Blistering, Justin Donnelly praised the album as "diverse, melodic and hard hitting all at the same time",[24] ranking it as one of Dream Theater's best releases.

[24] Billboard considered the results of the band's attempt to write shorter songs and use warmer instrumental textures to be "excellent",[23] particularly praising "The Root of All Evil", "These Walls" and "I Walk Beside You".

[23] Harley Carlson of MetalReview.com regarded Octavarium as "successfully [showcasing] the band's ability to craft emotive music,"[27] although noted that it is "unquestionably Dream Theater, yet there is something missing.

"[27] Vik Bansal of musicOMH praised "Never Enough", "Panic Attack" and "Sacrificed Sons", but dismissed "The Root of All Evil" as "Dream Theater by the numbers".

[28] Writing for Exclaim!, Greg Pratt praised the album's artwork and production, but said that "there’s nothing that blows any minds here, or even gives a mild bend; hell, a good chunk of this disc feels like basic radio rock from the local old-guy bar band".

[25] He noted that although there were some longer songs on the album, "this just feels like 76 minutes of overdramatic rock, too heavy on the light and weak, and not enough time spent on just showing off".

[24] Carlson stated that the album "draws closest comparison to Falling into Infinity and Metropolis II: Scenes from a Memory, but with far less complexity and edge.

[30][31] The main Octavarium world tour began in September in Finland, and saw the band continue with their "An Evening With Dream Theater" concert format.

[33][35][36] To celebrate the band's twentieth anniversary, the final concert on the tour was performed at Radio City Music Hall in New York on April 1, 2006.