"OMG" has a slow, trap production that is characterized by use of double hi-hat rolls, flute flows and a smooth bass.

In the lyrics, Cabello praises the appearance of her love interest while Quavo responds to her affirmation in his verses, where he also added a line from his Migos's song "Bad and Boujee" (2016).

The song was included as part of Cabello's set list for Bruno Mars's 24K Magic World Tour, where she served as an opening act.

Music critics complimented the track's club-friendly sound while others perceived Cabello's versatility for performing verses written by Charli XCX as well as her dynamic with Quavo.

On its first full tracking-week, "OMG" entered the top 100 on charts from several countries, including the U.S., UK, Scotland, Australia and Portugal.

[1] To achieve her desired sound, she collaborated with some notable producers, including the Norwegian production duo Stargate–composed by Tor Erik Hermansen and Mikkel Storleer Eriksen.

Together, they created some "emotional" and "deep" records like "Scar Tissues" to match the "Hurting" concept planned for the album, although while talking to XCX, Cabello had the idea to write something that was just "fun" and "not really take[n] too seriously".

[11] "OMG" is a hip hop-inspired song;[12] it comprises a trap drum beat with hi-hat rolls, flute flows and atmospheric bass beneath a "smooth", rap-like croon from Cabello and Quavo with auto-tuned effects.

[20] Praising Charli XCX's songwriting, Ross McNeilage of MTV UK felt that her "bold energy" can definitely be heard in the performer's verses, and considered the track a "cocky trap-pop banger for the clubs".

Mike Wass from Idolator, who regarded it as a "basic banger" in comparison to "Havana", while Arielle Tschinker from the same publication classified it as a "decent slow jam", although she perceived a lack of "sparkle and attitude" that Cabello had with Fifth Harmony.