Team Samui is summoned before the Fourth Raikage, and he and Karui argued over the possible reasons. He thought he was going to be scolded for putting mayonnaise on his potatoes. In actuality, they ar
e to deliver a letter to the Hokage about Killer B's recent capture. Omoi fears he may already be dead, but Karui insists he's being pessimistic. The Raikage tearfully vows that they will rescue B and both Omoi and Karui cry along with him.
As they near Konoha, Omoi worries that, once they got there, a girl would fall for him and, once he had to leave, she would commit suicide out of loneliness. After a short argument with Karui, she threw a rock at him. He then worries that the rock could cause an avalanche that would bury Konoha. Karui insists that he overreacts too much. When they arrive in the village they find that it is nothing but a crater, which Omoi blames Karui for. They ask around and find out that about the recent invasion of Pain.
ile Samui meets with the Hokage, Omoi and Karui overhear Naruto, Sakura, and Sai talking about Sasuke, B's captor. They demand all information they have on Sasuke, leading to a brief scuffle between the two groups. Omoi informs them of Sasuke's actions and promises to take vengeance against him which, if Konoha and Kumo are to be allies, they should help him with by providing information that can be used to kill Sasuke. Naruto assures them that there may be time to save B and offers to tell them everything they want to know. When it comes time do to so, however, Naruto can't bring himself to sell out Sasuke. He offers to let Karui beat him up, which she gladly does. Omoi watches for a time but then encourages her to stop, commenting that Naruto didn't seem to be the type of guy to rat out a friend - despite them being an enemy - which Omoi respected.
After reuniting with Samui, the team goes to the Konoha Archive Library to make a copy of all of Konoha's intel about Sasuke. As they set out to rendezvous with the Raikage, Omoi is glad to see that Karui is worrying about the injuries she gave Naruto. They deliver the information to the Raikage in the Land of Iron before returning to Kumo. When the later Raikage returns to the village with B, Omoi asks him about his new sword.
Omoi frees Tokuma and Ranka.
When the Fourth Shinobi World War begins, Omoi is placed in the Surprise Attack Division under the command of Kankurō. Omoi questions Kankurō's leadership abilities and experience considering they were around the same age, but Kankurō assures him that nothing will stop him from protecting his comrades and completing their mission. When they later set up a base camp, Omoi sets traps around their perimeter. These measures do not stop an Akatsuki attack, yet Omoi is quick to engage the hostile forces. He discovers that they are actually allies, forced to attack them by Sasori. Omoi releases them and, after getting them medical attention, regroups with the rest of his division.
The Ambush Squads battle.
Because Deidara is among the enemy's ranks, the division needs a way to deactivate his earth-based Explosive Clay.
Omoi volunteers to to try neutralising the clay with his lightning-infused sword. After he and Kankurō engage Shin who was being manipulated by Sasori and struggles to hit a moving target, Omoi is finally able to pierce Deidara after they are ruthlessly attacked by enraged Sai and is trapped in one of Kankurō's Black Ant puppets, deactivating his bombs.
Omoi, Sai, and Zaji are sent out to find other Akatsuki cells. They come across and attack a group consisting of Haku, Zabuza, Gari, and Pakura, but Sai's aerial attack of explosive tags fail and Haku attacks them using his Demonic Mirroring Ice Crystals. Before they can be killed the Third Division arrives to help. They later join the division protecting Kakashi while he executed his strategy.