WARNING: The following contains spoilers for Dragon Ball Super Chapter 66, by Akira Toriyama, Toyotarou, Caleb Cook and Brandon Bovia, available now in English from Viz Media.
Introduced at the end of Dragon Ball Z, Oob/Uub is the reincarnation of Kid Buu. While Oob played a big role in the now non-canonical Dragon Ball GT, he's taken a backseat in Dragon Ball Super, due mainly to the fact he's still quite young in the series. However, in Dragon Ball Super Chapter 66, Oob plays a key role in helping defeat Moro and save Earth.
After Goku accidentally let the energy-absorbing villain access the Angel Merus' powers, Moro involuntarily merged with the Earth itself, allowing him to drain the energy of anyone who touches his surface. While initially this puts Goku in the position of having to destroy his planet so as to save the galaxy, Whis soon reveals Goku can defeat the villain just by destroying the gem containing Merus' powers. Goku then attempts to attack Moro, but even though the Saiyan has access to Ultra Instinct Perfected, the villain proves too much for him, even with the help of Vegeta and the other Z-Fighters.
Moro soon grabs Goku and squeezes the energy out of him, causing the Saiyan to rapidly decrease in power and rely on lower-level transformations that aren't nearly powerful enough. Seeing Goku's dire predicament, Piccolo asks if Vegeta can use his Spirit Fission technique to give Goku energy. The Saiyan Prince believes that this is possible, and the Z-Fighters call on their allies -- except for Majin Buu, who is sleeping at The Lookout -- across the world to give up their energy to help Goku.
While they're successful in giving him their chi, the amount of energy Goku gets from his mortal allies proves insufficient, and he's only able to reach Super Saiyan Blue, which isn't nearly strong enough to best Moro. Goku once again starts losing power, and the Z-Fighters mull over fleeing Earth, though Kuririn notes this won't work, since Moro will eventually explode and destroy the galaxy. As all seems lost, a burst of powerful divine energy suddenly appears, shocking Vegeta.
Dragon Ball Super then shifts focus to a manifestation of the Great Lord of Lords/Grand Supreme Kai -- the being who originally imprisoned Moro and later merged with Kid Buu-- helping Oob give his energy to Goku. As Chapter 66 explains, when the Great Lord of Lords split, the divine part of his energy went to Kid Buu, meaning Oob inherited that power when he reincarnated. This divine energy proves to be exactly what Goku needs, and the hero unlocks a new, powerful transformation.
Although Oob has been a minor part of Dragon Ball Super, his intervention this time around proved vital to saving the Earth. While Dragon Ball GT is unlikely to once again become canon, Goku is sure to eventually train Oob, and this gift of power will certainly tip the Saiyan off to his future protegée's true power. This chapter also indicates Oob's power -- which is explicitly called divine -- may be even greater than in Dragon Ball GT. Oob could thus have a natural strength unlike anyone else in the history of the Dragon Ball franchise, meaning he's once again key to the series' future.
Dragon Ball Super Chapter 67 releases Dec. 20.