Pada 9 Januari, ONEUS secara resmi memulai debut mereka dengan merilis album mini pertama mereka ''Light Us'' dengan singel utama "Valkyrie".<ref>{{Cite web|url=https://www.soompi.com/article/1290011wpp/watch-oneus-makes-impressive-debut-powerful-valkyrie-mv|title=Watch: ONEUS Makes Impressive Debut With Powerful “Valkyrie” MV|accessdate=|author=|date=2019-01-09|last=|work=SOOMPI|publisher=|language=}}</ref> ''Showcase'' debut mereka digelar di YES24 Live Hall pada hari yang sama dengan perilisan album.<ref>{{Cite web|url=https://www.soompi.com/article/1290245wpp/oneus-opens-watching-fellow-produce-101-season-2-contestants-debut-first-veriverys-debut|title=ONEUS Opens Up About Watching Fellow “Produce 101 Season 2” Contestants Debut First, VERIVERY’s Debut, And More|accessdate=|author=|date=2019-01-09|last=|work=SOOMPI|publisher=|language=}}</ref>
Oneus merilis album mini kedua mereka ''Raise Us'' pada 29 Mei, dengan singel utama "Twilight".<ref>{{cite web|url=http://tenasia.hankyung.com/archives/1721507|title=원어스, 중세 시대 뱀파이어 연상되는 'RAISE US' 콘셉트 포토|language=ko|work=TenAsia|date=May 16, 2019}}</ref>
===2020: ''In Its Time'', ''Road to Kingdom'', ''Lived'', dan "Bbusyeo"===
Pada 10 Januari, Oneus menggelar jumpa penggemar untuk merayakan hari jadi pertama mereka yang bertajuk ''Oneus 1st Anniversary 'Our Moment''' di Yes 24 Live Hall Seoul.<ref>{{Cite web|url=https://n.news.naver.com/entertain/article/009/0004498645|script-title=ko:원어스., 데뷔 1주년 팬미팅 성료…더 기대되는 2020년|website=|via=Naver|date=2020-01-11|access-date=2021-03-12|language=ko}}</ref><ref>{{Cite web|url=https://n.news.naver.com/entertain/article/109/0004148806|script-title=ko:원어스, 데뷔 1주년 팬미팅 '대박 성공'..무대 천재+팬바보 입증|website=Osen|via=Naver|date=2020-01-11|access-date=2021-03-12|language=ko}}</ref>
