Mahiro Oyama was just a normal erotic-game loving dude...until he woke up one morning as a woman! Turns out his mad-scientist little sister, Mihari, tried out one of her new experiments on him... with a disastrous outcome, as far as Mahiro's concerned! But Mihari is as determined to study him as he is determined to go back to his shut-in, game-playing life, and one thing's for sure... life is going to get a whole lot weirder from here on out! (Source: Kodansha USA)

Oniichan wa Oshimai! is a web manga released on the author's pixiv account since February 25, 2017, and published in print by Ichijinsha since June 27, 2018. The web manga has also been published as doujinshi since May 6, 2017, when the first volume was released at COMITIA 120. The series has been published digitally in English as Onimai: I'm Now Your Sister! by Kodansha Comics USA since August 31, 2021.