Ōmoto was born and raised in Fukuyama City, Hiroshima Prefecture, and attended Hiroshima Actors School with friends and Perfume members Ayaka Nishiwaki and Yuka Kashino. She was placed in the advanced class for singing, as she was always interested in singing. In a letter she wrote to her "Future Me" she recalled her singing teacher telling her if she just wanted to sing and dance because she loved it she was not fit to be a singer. Nishiwaki and Kashino were still in the beginner class at this time, meeting Ōmoto secretly.
She joined the group in 2001, replacing the original third member Yūka Kawashima, who left the group to focus on her schoolwork. In 2002, she appeared in an advertisement for Johnson & Johnson.