"O" is a song from American R&B singer Omarion. It was released on July 17, 2004 as the first single off his debut album of the same name. "O" was the first choice for Omarion's debut single in the UK and was picked over "Touch", which was the original choice for his first UK single. The song was Omarion's first hit as a solo artist, peaking at number 27 on the Billboard Hot 100 and peaked at numbers 24 and 12 on the Mainstream Top 40 and Hot R&B/Hip-Hop Songs charts respectively. The song also charted in New Zealand and the UK, peaking at numbers 18 and 47 respectively.

"O" debuted at number 68 on the Billboard Hot 100 the week of January 22, 2005. Eight weeks later, it peaked at number 27 the week of March 19, 2005 and stayed there for two weeks. It stayed on the chart for twenty weeks.

Directed by Chris Stokes (who previously directed Omarion in the 2004 film You Got Served), the video features Omarion going over to a girl's house and getting intimate with her. Intercut are scenes of Omarion dancing on the roof of an apartment with his friends, a woman or by himself. The video won the BET Award for Viewer's Choice at the BET Awards 2005.

Omarion first performed "O" and "Touch" at the 2005 BET Awards on July 28, 2005. The performance was Military-themed with Omarion and the dancers dressed in Army outfits. He performed the song again at the 2005 American Music Awards on November 22, 2005 as part of a medley with Bow Wow's "Like You" and "Let Me Hold You".

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