Onii-chan wa Oshimai! is a comedy and slice-of-life anime series that follows the life of Mahiro Oyama, a shut-in who suddenly wakes up one day to find that he has been transformed into a girl. The story humorously chronicles Mahiro's struggles and adventures as he learns to navigate his new life and identity, with the help of his younger sister, Mihari. The series explores themes of self-discovery and personal growth, all wrapped in lighthearted humor.