Oniichan wa Oshimai!

Self-professed "home security guard" Mahiro Oyama has not left his home in years, secluding himself in his room playing erotic visual novels. This depraved lifestyle causes his prodigious sister, Mihari, to worry about his well-being. In hopes of solving this problem, she devises a plan to rehabilitate him back to normalcy.

The first part of Mihari's plan is to concoct a medicine that changes her brother's biological constitution into a bona fide female, much to Mahiro's vehement dismay. Stuck in this predicament, Mahiro has no choice but to live out his life as a cute girl until the effect wears off—if it ever does.

[Written by MAL Rewrite]

Oyama, Mahiro Main

Kouno, Marika Japanese

Oyama, Mihari Main

Ishihara, Kaori Japanese

Hozuki, Kaede Supporting

Kanemoto, Hisako Japanese

Ouka, Asahi Supporting

Yuuki, Kana Japanese

Hozuki, Momiji Supporting

Tsuda, Minami Japanese

Murosaki, Miyo Supporting

Hioka, Natsumi Japanese

Fujimi, Nemu Supporting

Sakurai, Mia Japanese

Leonardo Supporting

Uchida, Yuuya Japanese

Nagase, Miyako Supporting

Oda, Karin Japanese

Kitahara, Yuriko Supporting

Nagae, Rika Japanese

Ootomo, Toshiya Producer

Hishiyama, Mitsuteru Producer

Hatakeyama, Takurou Producer

Higashi, Mao Assistant Producer

Reviews

Jul 10, 2025

Necris

Recommended

In the beginning I was worried if sex change motive will be annoying or boring for me - so after watching through the entire season - his sex change is only a hook and it lost it's importance after first few episodes. Mahiro is bona fide 100% girl in both looks and character that has zero behaviours that would even indicate her history as a male. At first I tried to explain how skittish she was by her being a NEET, but the longer the season went, the more obvious it became, that it is not the case.

So if we ignore sex change, what ... did we got in the end? A pretty good CGDCT anime in which one of the characters is new to the "Girl Things". If we look at it in this category, then I really can't complain about anything. It has all that makes anime in this genre successful:
- Beautiful and plenty fluid animation
- Fun and engaging characters
- Not bad comedy
- Good music, especially really fun opening and ending.
- And what is most important for me: a good tempo to the story. I hate when anime feels rushed, and it is especially jarring in relaxing CGDCT genre.

As such I can wholeheartedly recommend watching Onimai

Reviewer’s Rating: 8

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Dec 18, 2024

useless-cheese

Mixed Feelings

This Series does not adapt all the chapters and it adapts them in an odd order. This anime is fun and good but like horimiya just read the manga and get a fuller picture.

if you see this review after wards you only missed chapter 4(very trans), 19, and 23, along with most bonus chapters/pages

The anime itself is still good, with a bubbly art style that makes our cast look good. the humor in this is good for an anime but what makes this series any fun is watching the characters interact. The main cast are all so charming and distinct.

Reviewer’s Rating: 7

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Dec 11, 2023

Smilomaniac

Not Recommended

The premise is from the get-go that a man (older brother, true gamer nerd/enthusiast, recluse but ultimately harmless) wakes up having been turned into what looks like a pre-teen girl by his "mad scientist" sister, via a drug. In her room she has a manual on her desk which deetails how she wants to turn him into her little sister, meaning this is entirely deliberate with an agenda.

It SOUNDS like this should be a hilarious comedy to give a guy what he deserves in order to appreciate what being a girl or a woman is like, as punishment.
That's not at all what it is. ... This is entirely about a selfish wish from the sister's side to get rid of her nerd brother and gain a cute little sister that she can dress up and take care of and the brother-turned-sister just goes along with it for no good reason with no strong opinions one way or the other.

It's dressed up as being wholesome and shows some awkward situations because it's a gender-bender, but really it just seems like a small girl's coming of age slice of life with questionable fan service. Gotta ask, why isn't that just the story?

So you're left wondering where the plot is going and why you're looking at pre-teen panty shots. Where's the joke?
If you think about the premise a little, this is a creepy story about a woman who abuses her sibling and psychologically manipulates them, besides the fact that she drugged him with what seems like permanent side effects.
Again, this could've been interesting, she could've been a genuine psychopath who mentally tortures her brother, but instead it's played off completely straight and even lovingly.

Moral of the story? It's better to be forcefully gender transitioned and socially groomed than be a male recluse.

You tell me how this isn't wildly thoughtless and even offensive.

I'd like to touch on another thing which is the topic of being transgender in the 2020's.
How odd and insensitive it must be to watch someone with no interest in becoming the opposite sex just get a pill or some liquid and magically transform with no effort, no pain or any cost.
I'm not an "ally", I don't particularly think encouraging being trans is at all healthy or the right thing to do, but I can't help but feel that this show has to just twist the knife for a person who wants to be the opposite sex and feels genuine grief with who they are.

It's not the show-maker's responsibility for how it makes people feel, but I think it's another thoughtless and irresponsible way of approaching what could've been a good and interesting story.

It's not impossible to view past its flaws and just enjoy a 'wholesome' story with a bit of ecchi, but man... this is in poor taste.

Reviewer’s Rating: 4

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