Oiwa was born in Noda, Chiba on 20 November 2001. She began rhythmic gymnastics at age five and quit playing piano at age six to focus solely on rhythmic gymnastics.[3] She is a student at Kokushikan University.[4]
Oiwa competed at five events during the 2019 World Cup series. First, in Pesaro, she finished seventeenth in the all-around.[9] Then in Baku, she placed twelfth in the all-around.[10] She qualified for the clubs final at the Guadalajara World Challenge Cup and finished sixth.[11] She then finished twelfth in the all-around at the Minsk World Challenge Cup. Then at the Kazan World Cup, she finished eighth in the clubs final.[12] At the 2019 World Championships in Baku, she finished nineteenth in the all-around final and eighth in the team competition with Sumire Kita and Kaho Minagawa.[13][14]
Oiwa competed in the 2021 World Cup series to qualify for an Olympic berth. First, in Sofia, she finished eighth in the all-around and seventh in the ribbon event final.[15] Then in Tashkent, she placed seventeenth in the all-around.[16] She then finished nineteenth in the all-around at the Baku World Cup.[17] Finally in Pesaro, she finished tenth in the all-around, eighth in ball, and fifth in clubs.[18] These results earned Oiwa an Olympic berth. She was the third-highest ranked eligible gymnast, after Slovenia's Ekaterina Vedeneeva and Uzbekistan's Sabina Tashkenbaeva.[19] At the 2020 Olympic Games in Tokyo, Oiwa finished nineteenth in the qualification round for the individual all-around with a total score of 87.550.[20] After the Olympics, she competed at the World Championships in Kitakyushu and finished thirteenth in the all-around final with a total score of 93.400.[21] She also finished fifth in the team competition with the Japanese team.[22]
^"大岩千未来、200度以上曲がる"爪先美人"がめざす東京五輪の表彰台" [Chimi Oiwa, aiming for the podium of the Tokyo Olympics with a "beautiful toe" that bends more than 200 degrees]. Hochi (in Japanese). 29 May 2018. Retrieved 31 July 2022.
^"大岩選手が日本の五輪出場2枠目を獲得 新体操W杯ペサロ大会" [Oiwa wins Japan's second Olympics Spot at Rhythmic Gymnastics World Cup Pesaro]. Kokushikan Sports (in Japanese). 1 June 2021. Retrieved 31 July 2022.