Oshima attends the same island high school as Kanon Miyamori. She nurses a crush on Kanon's ex-boyfriend, Kojima, actively pursuing him despite his clear disinterest. This unrequited pursuit positions Oshima as Kanon's perceived rival for Kojima's attention, fueling friction within their group.

Her initial confidence and efforts to attract Kojima fit an Alpha Bitch mold. Yet this bravado conceals deep vulnerability. A confrontation reveals that even during her brief relationship with Kojima, his conversations constantly centered on Kanon, leaving Oshima feeling inadequate and perpetually overshadowed. This confession exposes her hidden pain in loving someone fixated on another.

Oshima's experience with rejection and her emotional honesty become pivotal for Kanon's growth. She directly challenges Kanon's hesitancy and half-measures in relationships, urging her to confront her own feelings and responsibilities as the Priestess of the Sky decisively and without reservation.

Her personal struggles with unrequited love and self-comparison mirror broader themes. During the island-wide emotional crisis fueled by Sedna's influence, Oshima's suppressed sorrow and jealousy erupt alongside others' negativity, demonstrating how her pain contributed to the collective darkness forming Sedna.

Oshima acts as a narrative foil to Kanon, contrasting Kanon's initial emotional withdrawal with Oshima's outwardly proactive, yet equally troubled, quest for romance. Her journey underscores the complexities of teenage relationships and the sting of unmet expectations within the series' exploration of human emotion.