WARNING: The following contains spoilers for the One Piece manga, available in English from Viz Media.
The One Piece manga's still ongoing raid on Onigashima has been an uphill battle from the get-go for the Alliance. However, thanks largely to O-Tama's Devil Fruit (the Kibi Kibi no Mi) ability, the tides of the battle are slowly beginning to turn. But what exactly is the Kibi Kibi no Mi, and how could the young girl be the key to the Alliance's victory over the Beast Pirates?
O-Tama was first introduced in Chapter #911 of One Piece. Following Luffy's arrival to the Land of Wano, he encountered a team of Beast Pirates, including a large baboon fighting a lion-dog. After an initial skirmish, Luffy discovered that the pirates had captured a young girl, who revealed herself to be O-Tama, the lion-dog's owner. While Luffy had defeated the pirates, the baboon continued to fight the animal. However, after feeding the baboon a millet dumpling she retrieved from her cheek, O-Tama tamed the baboon instantly. Despite his typically slow-witted nature, Luffy immediately identified this as a Devil Fruit ability.
The Kibi Kibi no Mi grants its user the ability to produce small millet dumplings by pinching their cheeks, also referred to as kibi dango, instantly taming animals when fed to them. The user of this ability must choose to produce the dumpling themself, as seen in Chapter #916 when Holdem, a headliner for the Beast Pirates, attempted to create the millet dumplings himself by repeatedly pinching Tama's cheeks with a wrench. This only hurt O-Tama as she did not want to hand over any dumplings to the Beast Pirates.
In addition to taming wild animals, the Kibi Kibi no Mi can tame SMILE Fruit users. Due to the artificial Devil Fruits giving their users traits of various animals, they become susceptible to the kibi dango. This is first demonstrated in Chapter #918 when O-Tama feeds a dumpling to the Beast Pirates Headliner Speed, a horse SMILE Fruit user. The dumpling's effects appear so strong that Speed was even willing to fight Kaido to defend O-Tama. However, in Chapter #1011, Usopp confirmed that the kibi dango's effects do not apply to natural Zoan users such as Kaido and the other upper members of the Beast Pirates.
At the beginning of the raid, the Alliance arrived with 5,200 soldiers, facing off against an army of 27,000 between the Beast Pirates and members of the Big Mom pirates present on Onigashima. Throughout the raid, these numbers have changed drastically.
CP0, who appeared to be monitoring the invasion from afar, gave the audience an update on how many forces each side had in Chapter #1016. Due to Beast Pirate All-Star Queen infecting many of his troops with a deadly virus, many of the grunt-level forces of the Beast Pirates chose to switch sides. After some casualties on both sides, the Beast Pirates were left with 20,000 soldiers against an allied force that had become 7,000 strong.
In Chapter #995, O-Tama appeared on the battlefield, saving Nami from Ulti. Since then, she's been able to feed each SMILE user of the Beast Pirates a kibi dango. In the latest chapter, Chapter #1017, O-Tama ordered all SMILE users who had eaten her dumplings to switch sides, amounting to another 500 soldiers. This leaves the Beast Pirates with 19,500 against a force of 7,500.
While the Beast Pirates still have the advantage of superior numbers in One Piece, the growing number of allied forces indicates that the Alliance is on the path to victory. The raid on Onigashima is far from over, and thanks to O-Tama's abilities, the allies have a much greater chance of success.