[8] As part of the promotions of their first EP, the group performed on music shows for seven weeks, held a 4-hour fansigning event, and worked as baristas in Seoul, Busan and Gwangju.

[10] On October 4, the first teaser for their second mini album Line Sun Goodness was uploaded to YouTube, announcing that their first comeback would take place in the same month.

[11] Due to technical difficulties, the music video for the title track "Sage" was not released until three days later, on November 2.

[12] The song explores, both in the lyrics and through the visuals of the music video, temptation and redemption, and the mythology and characters behind OnlyOneOf's concept, which has been developed by creative director and producer Jaden Jeong over three years taking inspiration from religion and the Bible.

[22] The three-track single album was released on May 21 along with the title track "Angel", an EDM and hip-hop song by Gray,[23][24] who declared he had wanted to collaborate with OnlyOneOf for the traditional K-pop atmosphere and elegant vibe of their music.

[31] Overall, the whole album explores the theme of instinct, and deals with the sexy concept Jaden Jeong had in mind when OnlyOneOf was created.

[32] The members touching each other's bodies and using strings in the choreography caused a controversy for being too suggestive as soon as they performed "Libido" on M Countdown on April 9, to which OnlyOneOf responded that it emphasized the theme of the music and that they hoped the public could focus on the overall aesthetic and expressive power.

[35] At the end of the year, "Libido" was chosen as the best Korean music video of 2021 by Rolling Stone India for breaking boundaries and representing the LGBTQ+ community.

[41] On September 10, the group unexpectedly released the single album Unknown Ballad 2.3, containing the song "Mono" written and composed by Nine.

[59] After a pause for the group's second Japanese single album Cunning Woman (ズルい女) – a remake of the 1995 song by Sharam Q – and a three-stop tour in Japan at the beginning of October for the OnlyOneOf 2nd Japan Live 2022,[51][60] the Underground Idol project continued: on October 27, Rie released "Because",[61] followed by Mill on November 28 with "Beat".

[67] The album tells the stories of young people hurt by others wandering in Seoul; Nine and KB participated in writing, composing and arranging the seven songs.

[70][71] On December 14, 2023, they announced their comeback with their sixth mini album, Things I Can't Say Love, released on January 10, 2024.