Anguished Declaration of Love: Subverted in a chest-stabbingly cruel scene. Kitahara is forced to "confess her love" for Pizzata by her Harada and Sugawa, and then soon by the entire class cheering for it. This later gets painful for Pizzata too, as he starts having (unrequited) feelings for her.
The Atoner: Kurosawa after admitting to the class what he did, and getting bullied for it.
Dawson Casting: Unusually for a slice-of-life series, it doesn't take place in high-school, but in middle-school. It's awfully easy to forget.
Deconstruction: Very much so, of the typical Ecchi "protagonist is a pervert but that's ok" genre. The hero is a pervert, but it's considered a horrible thing, he grows out of it and his life vastly improves because of it, he takes the blame for what he did voluntarily and everyone has a realistic reaction, the tortured girl snaps realistically after years of bullying, including several off-screen suicide attempts, being a Hikikomori is considered a very bad thing, etc., etc.
Although Kurosawa is still a pervert even after the heart change and facing most of the consequences for his actions, he still masturbates and fantasies about schoolmates in college High School (just privately). But he stopped being a antisocial, misogynistic and egoistical reject.
Yes, the problem was much moreso the fact that Kurosawa was very cynical loser and thought very little of everyone else, which is pretty much why any female he knew wouldn't be anything more than a sex object to him, if they were attractive. The whole point of the story is about him realizing that he's wrong, that people are good and having relationships with them is worth it; masturbation was just a metaphor for antisocial behavior in general.
Despair Event Horizon: Learning that Takigawa and Nagaoka are going out together was already a pretty bad blow, but doing to her what he did to the other girls pretty much disgusted him from everything. He gets better, thankfully.
Easily Forgiven: Mostly averted. After he confesses, Kurosawa has to put up with constant harassing and his newly earned "title" follows him to high-school. Takigawa ends up forgiving him, but even her is still shocked for a while and has trouble actually saying it out loud. Played straight with Nagaoka though, but that's in his natural personnality.
Genius Bonus: "Onani" meaning masturbation is a literary one, as the oldest known reference to the "sin" of masturbation is Onan in The Bible.
Genre Shift: Starts off as a Death Note parody/ecchi crossover with lots of fapping with a slight dark edge in regards to the bullying but a hint of fluffy romance... and then the fluffy romance turns into a crushing love triangle, the main character getting worse and worse until he realises what he's done and fully admits his crimes causing his rather horrifying and realistic atonement, the first bullying causes the victim to snap and become a suicidal recluse. But they all get better.
Heel Realization: Kurosawa, upon seeing Takigawa's painting. He confesses to his actions soon after.
Hikikomori: Kitahara becomes one after she snaps. It's not played for laughs nor is it handwaved — it's mentioned that her mother has had to stop her from committing suicide multiple times, and she's openly terrified for her daughter's safety when Kurosawa comes to visit.
Hurricane of Puns: Perhaps the oddest example in fiction. It really isn't one in the original language, but when you translate it into English, it becomes a "Coming" of age story about the "Master of Masturbation."
If I Can't Have You: What motivates both Kurosawa and Kitahara to "punish" Takigawa for getting in couple with Nagaoka.
Important Hair Cut: A couple of them - Takigawa before she entered middle school, and Kitahara at the very end of the series. You can also tell that Takigawa really is dating Nagaoka when you see her come to school essentially cosplayingTsuruya.
Japanese Honorifics: Nagaoka calls everyone "dono" ("Mister/Madam" or "Lord"). Yes, even his girlfriend.
Karma Houdini: Nobody ever finds out that Kitahara was involved in the incidents; although of course she suffers plenty from the bullying nonetheless.
Reverse Cerebus Syndrome: Right from the moment Kurosawa sees Takigawa's painting, the series gets much less cynical and its main character much more likeable. Although you could also say metting Takigawa was the start of his development.
Serious Business: Kurosawa takes fapping very seriously, going so far as to call it his "duty".
Sliding Scale of Idealism Versus Cynicism: As it starts out as a Death Note parody, it begins pretty far on the cynical end of the scale, starting of with a showcase of humanity at its most dickish, misanthropic, and cruel. It ends far, far on the other end of the scale, with an unflinchingly optimistic message about taking the high road, friendship, inner strength, and forgiveness, with even Kitahara, after all the hell she goes through and inflicts, making it through her shut-in phase with Kurosawa's support.
The series' title, specifically the word "onani" due to different modifications to Hepburn Romanization and the debated status of the proper romanization of elongated vowels sounds. The dropping of the long vowel is the most common interpretation amongst the fanbase.
Takigawa has her last name constantly misspelled as "Takagawa".
Third-Option Love Interest: So does Kurosawa end up with Takigawa, who he had a crush on, or Kitahara, the one he eventually defrosts? Neither of them. He actually ends up with Sugawa, the bully and his first victim, since by the time they start getting together, she has become much less mean and more Tsundere.
Wham Episode: When Takigawa and Nagaoka are revealed to be dating. The cries and shouts of unsuspecting new readers can still be heard in various sites to this day. The series just takes a completely different turn from that point on.