"Octavarium" reached the top five in the Finnish, Italian, and Swedish charts, as well as the top ten in the Dutch, Japanese, and Norwegian charts. Critical reception of the album was generally positive. Writing for Blistering, Justin Donnelly praised the album as "diverse, melodic and hard hitting all at the same time", ranking it as one of Dream Theater's best releases. He particularly praised the title track, considering it to be "another Dream Theater classic". "Billboard" considered the results of the band's attempt to write shorter songs and use warmer instrumental textures to be "excellent", particularly praising "The Root of All Evil", "These Walls" and "I Walk Beside You".