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Oga is first introduced as a strong fighter, beating up some fellow class mates and then making them bow down to him because they had attacked him in his sleep. It is revealed that he is relaying the story of a series of strange events which leads to him becoming the Earthly father of the son of the devil, Beelzebub IV. He was picked to become Beelzebub's father because, in the eyes of Alaindelon (see below), who was carrying the baby, he appeared to possess the qualities of the ideal parent for the future Devil King: strong, arrogant, and thinking nothing of fellow man.
Oga and Beelzebub (nicknamed baby Beel) cannot be separated by more than 15 meters or else it may result in instant death for Oga, according to Hilda (as of chapter 38, the distance has increased by 8 centimetres). A tattoo-like seal on Oga's right hand, which grows larger the more he fights, signifies the contract between Beelzebub and him, whereby Oga can access large amounts of demonic power. He tries to stop fighting to limit the powers which will ultimately be used to end the human race. However, he is unsuccessful, as his fame of invincibility keeps attracting the attention of other delinquents who seek to conquer the fame that would come along defeating him.
As the stories progresses, the reader learns that Oga is not exactly like the other delinquents in Ishiyama High School. He is certainly a hoodlum, as he relishes the opportunity to fight and seems to take some pride of his reputation of being impossible to beat. However, he is not a bully, and does not use his strength to intimidate other people. Also, at first he does not seek fights actively, is not interested in the power struggle that pits Ishiyama's gangs against each other, and does not want to belong to a gang or find underlings of his own. He minds his own business unless provoked, which to his dismay happens all the time.
However, when he learns that the baby will become attached to someone that is both stronger and possessing of the attributes that make the ideal parent, he decides to actively seek confrontation with delinquents who show promise in this regard. This first pits him against the Touhoushinki (see below), but he is disappointed that none of them is able to defeat him. In the process, baby Beel becomes progressively more attached to him and Oga's access to demonic power becomes easier. On defeating Himekawa (see below), he uses for the first time what later becomes his signature attack, the Zebub Blast: a powerful outburst that is strong enough to destroy a building. Later, he becomes able to draw demonic power even without physical contact with baby Beel, and by chapter 84, he uses it consciously for the first time.
Oga's skill is revealed, in the course of the story, to be the consequence of a strong fighting instinct honed by numerous confrontations with other delinquents from an early age. His fame begins at Kata Middle School, when it is told that the mean look in his eyes invited attacks from older students, from which he always came out on top, earning him his first nickname, Mad Dog Oga (by Furuichi). At the end of his 7th grade, he fought alone, and presumably won, against about fifty thugs who had the school under siege.
His exploits at Kata cemented his reputation and at the time of his arrival at Ishiyama as a first-year student, Oga is already known by aliases such as Demon and Raging Ogre, seems to have a lot of enemies from the outset, and to make even more as the series progresses (despite trying not to fight). He also seems to have no friends except for his classmate, Furuichi. He lives with his parents and elder sister (who is an ex-delinquent; it is revealed that she was the first head of the Red Tail gang), though Hilda and Beelzebub have recently moved into his house.
Surprisingly, and often with comical effect, Oga is a decent father figure to Beelzebub. He appears to have his own skewed idea of what a 'man' should be like, and attempts to pass on these ideals to Beel. Despite being called evil by almost every character of the series, he has shown to have a calmer and - if only slightly - warmer side to him, such as when he speaks with Hilda about his position as Beel's parent and assuring Lamia that if Beel ever got ill again he would call her immediately. Despite spending much of the first arc trying to pass Beel on to someone stronger and more evil than him, after all the time they spent together they've formed a close bond, and Oga seems to act more friendly towards Be'el.
His long fighting record has made him a formidable enemy: although he has no formal training in martial arts, he is quick enough to dodge attacks from skilled fighters like Kunieda or Shinjou Alex; his agility counterbalances Toujou's heavier build and allows Oga to defeat him; and his resilience enables him to resist most direct hits from Kunieda (and later Miki) while only sustaining minimal injuries. Oga is possessing of incredible, near superhuman, strengh, even if not heavily built: he is able to send people flying in the air with an effortless punch; most of his opponents are knocked out cold with one hit, often being planted headfirst into the ground, ceiling or walls for comic effect; and in chapter 91 he lifted an enormous boulder from the ground. He comes out of most fights unscathed; and the number of opponents, or how they are equipped, seems to be of little concern to him.
He is able to hold his own against a demon for some time without resort to demon power (although he did use the Zebub blast against this enemy, it had no effect). He can feel Saotome using his demon power, which shows he has notable strength for a human. In chapter 75, the Zebub spell temporarily extends up his arm, over his trunk and across his face, giving him a demonic appearance in the eyes of Kiriya. Later, though, upon having to be rescued from a confrontation with a demon by Saotome, he becomes irritated by the fact he is unable to protect anyone and decides to take formal training from Kunieda's grandfather. After teaching Oga the basics, he engages Saotome to instruct the young man on how to tap Beel's demonic powers to their full extent. In chapter 106, he shows his acquired skills for the first time by defeating a demon with a new movement, the Zebub Emblem: after placing a projection of the Fly King Seal onto the intended target, Oga can launch a barrage of punches that cause an enormous amount of energy to accumulate on the victim, later released as a massive explosion. Oga also becomes capable of dissipating the dark aura of energy that demons release when they prepare an attack.
Also as a consequence of his training, Oga develops an extreme attack, comically dubbed Super Milk Time, whereby he drinks Baby Beel's milk as a way of increasing their synchronisation ratio, thus enabling Oga to use Beel's power without limit. As a result, he can unleash a frantic, wild attack that can overwhelm even Saotome. As a trade-off, Oga and Beel's mentality coalesce and Saotome has warned Oga that allowing himself to run on this state for longer than five minutes may overtax him physical and mentally to the point that he will lose his humanity.
As most thugs in the series, he lacks any particular technique in fighting, mostly relying on his natural prowess to diminish the advantage of trained martial artists like Kunieda and Miki. However, he has figured out some classical techniques from wrestling: he defeats Toujou with a suplex and leads baby Beel in a Clothesline, an Elbow drop and a Boston crab. Later, not wanting to be looked down on by Miki (see below), he starts to invent nonsensical names for his punches and kicks, so that they will sound more respectable like the elaborate movements of trained fighters. However, his immense strength and agility mean that even basic techniques may become quite destructive in his hands. So far, he has learned one specialized movement, Nadeshiko, from grandpa Kunieda, and in only one day's training became capable of using it to split massive rocks with ease.
Oga is also pretty honourable by delinquents' standards. Although he has no qualms about humiliating petty thugs who pick fights with him, e.g. by making them bow down to him, he respects his opponents of more consequence. In the case of Toujou, for example, after having won the fight against him, Oga is willing to have everyone leave the scene quickly to let Toujou brood over his defeat in peace. He is also quick to acknowledge other people's strength or courage, which, for example, is one of the reasons Miki admires him. Oga is easily agitated and does not avoid fighting. However, he also seems to care, at least to some degree, about his classmates, as he quickly stopped Nene when she was about to fight Miki, telling her she would lose if she did. Oga, along with Furuichi, is an avid fan of the video game series Dragon Quest, and regularly makes references to the series in the manga. In a twisted manner, Oga is able to show some affection for his friends. For example, he misses Furuichi during holidays and uses Alaindelon to kidnap him from a five-star resort, if only to annoy him. He also resorts to the extreme measure of badly beating Miki up in order to prevent him from taking part in a fight against Kiriya and his gang.