Oshima passed the first AKB48 audition in December, 2005 along with 23 other girls. She was assigned to Team A, and has participated in recording of most of AKB48's single. In April 26, 2007, it was announced that her contract would be transferred to Horipro. Upon joining the new talent agency, she was put on the roster of XANADU loves NHC, a futsal team consists of idols from Horipro; her jersey number is 18. Oshima made her first appearance on Kouhaku Uta Gassen, an annual music show where the most successful artists compete, with AKB48 on December 31, 2007.
On March 24, 2008, she participated in the opening party of Channel Café, a channel broadcasting simulator event, at Roppongi Hills, Tokyo. From April 2008, Oshima will appear regularly on Movie Plus navigating show, Plus Style.
On February 21, 2009, she made public that she graduated from AKB48, and, on April 26, 2009 she graduated from the group with the last performance in the NHK hall.