Ōu was originally founded by Riki, who later passed the title to his son Gin. Later on, Gin passed his leadership to his own son, Weed. But later Weed was resigned and passed his leadership to his second oldest son Orion.
The Ōu Army was founded by Riki out of his hatred towards Akakabuto, a monstrous bear who began terrorizing the peace in the mountains. The pack initially had about 70 dogs, divided into 12 platoons of former hunting and pet dogs. To fight against Akakabuto and the bear's army, Riki sent out his platoons to gather more fighters from all across Japan.
By the start of the final battle, the Ōu Army consisted of 1,800 dogs, though only 750 of them survived to face Akakabuto.
As Riki dies from his wounds during the fight, he passes on the leadership of Ou to Gin, urging him to finish the battle.
After Akakabuto was defeated, most fighters who survived returned to their homes, though many also stayed as loyal allies and members of the Ōu Army.
During the Wolf Arc, Ōu had at least 700 soldiers.
Most of the Ōu fighters and their children settled in the mountains of Ōu, ready to fight if needed.
After humans attacked Ōu, only about 15 dogs were left. They gathered more soldiers with Weed in charge, in hope of rescuing Gin who was a prisoner of Hōgen and to prevent Hōgen from becoming the ruler of Japan. Weed gathered 72 soldiers to fight Hōgen, which later increased to 100 with the help of Moss. After Gin escaped, he gathered an army of 500 pet dogs. Later, Hōgen's platoon leaders Buruge, Batto and Kite joined Ōu army, leaving Hōgen alone against an army of around 1000-1050 dogs. Hōgen was eventually defeated by Weed alone and again many soldiers returned back to their home areas.
Sometime after Hougen's defeat, Weed and his friends head south to find Jerome but in the midst of their search they come across a pack of dogs lead by Yukimura who were at war with a tribe of Japanese macaques lead by Shogun. During the battle the two leaders find out they are brothers even though Shogun is defeated Yukimura also faces his demise but the Japanese macaques are free. Weed comes across two dogs named Kotetsu and Koyuki to his amazement who looks his deceased mother Sakura as the two begin to spend more time together they fall in love.
However, Gin tells Koyuki the reason why his son is hanging around her because she looks so much like his mother she becomes heartbroken knowing she doesn't deserve to be with him. Weed hears that Koyuki is being taken away to be a breeding dog and he goes off to rescue her knowing that he loves her for her and not because she looks like his mother with that settled Gin and the others agree to the pup's love.
Jerome makes it all the way to Hokkaido the territory of Gin's old friend Hakuro who informs him that the role of leadership has been given to Gin's son Weed. But Hakuro warns the shepard that war dogs from Russia have invaded and Jerome comes across the Russian Army as well as mysterious female named Lydia. Jerome manages to escape the dogs but not before he sees the damage they have done and warns Hakuro but ends up as a prisoner of the Russian Army Dogs again.
Despite things turning into a full scale war, Hakuro, his pack and three of sons have been slain all except for Suzaku and his family who manage to escape and informs the Ohu army what happened. Gin leads the army away to help during the war the Ohu Army has gained two new allies from the Russian Army Maxim and Lydia who has started to fall in love with Jerome. However, the Russian Army has a traitor named Viktor who wants to conquer everything for himself. With Viktor defeated Maxim leads his army back to Russia and Weed leads his back to Ohu
Everyone returns to Ohu but this time they meet a young pup named Koshiro who is the son of Weed's lost brother Joe. But not everything is on happy terms with the brothers because Joe still blames their father Gin for what happened to their mother. To make matters worse Joe and his pack are having trouble dealing with a group of hybrid bears that are terrorizing his home, so Joe makes a deal after all the hybrid bears are beaten he'll return home to see their father.
The battle against the bears has been long and hard and during the fight Weed's friend GB loses his life but dies as a hero saving his leader. In anger, Weed kills the final hybrid bear but the duo fall into dam the bear finally dies but Weed is no where to be found believing that Weed is gone too. Joe assumes the role of leader fulfilling their agreement but to everyone's relief Weed returned alive and they all return to Ohu. Sometime later, Weed becomes a proud father to his own children with Daisuke naming them after the star constellations in the night sky.
The baton of the Ohu Army has been passed on yet again, this time with Weed's children Sirius, Orion, Rigel and Bellatrix. This time the pups and their allies deal with natural disasters thanks to earthquakes and volcano eruptions the army gets scattered but most of the soldiers are alive.
The four siblings end up separated from each other going through adventures of their own. With one threat over another is on the rise Masamune and his clan wish to conquer all of Japan starting with Ohu and it's up to Sirius, Orion, Rigel, and their friends to stop him.
With Masamune defeated everyone heads back to their lands and the pups experience their first spring.
Monsoon, the son of Akakabuto, has returned to reclaim his father's territory. Along with his kin, he attacks the unsuspecting army and in the process, badly injures supreme commander Weed. As of chapter 19, the entire Army is badly injured and unable to fight, some have been killed. Paradise falls and Monsoon claims the land as his own.
The dogs refuse to give up but, with Gin, Weed and now Unsai injured and the two eldest sons of Weed bickering with each other who are left to lead the army now. Unsai's right-hand takes command of the army, meanwhile Orion disowns Sirius as his brother, now the dogs can do very little against bears. But with the arrival of humans, the dogs are reawakened with courage and hope of defeating them once and for all. Then, as of chapter 97, all of Monsoon's Kin are defeated, but Monsoon is still alive but injured. He disappears into the mountains, leaving his dead kin behind.
The dogs' victory is short-lived, as first it was Weed, then Gin, then Unsai and now Akame are all injured and yet alive but they can't continue the fight.
Now Kyoshiro is in charge of the army, and he wants no more fighting among the brothers as the army watch Akame and Izou leave. Shion and Sirius leave on their own to fulfill Akame's last request.
With Monsoon temporarily gone, Daisuke and Hidetoshi lead the villagers in the process of healing the injured dogs and a burial memorial of the fallen dogs.
The pups start to become of age, the Pet Dog Squad begin to make their journey home and the injured soldiers start to make good recoveries including Weed, Gin and Akame. While Sirius searches for Monsoon; Orion and Rigel visit their father Weed.
After explaining to Weed and Gin what has happened, despite having not been fully recovered Gin ventures back to Gajou to check on the pack. But nightfall they discover that the Kamakiri brothers and Orion's friends are gone knowing that they went to rescue Sirius before he makes a foolish mistake. Rocket leaves with Rigel to bring them back meanwhile with everyone out of commission including Gin who left with Daisuke so his recovery could be watched. The role of leadership is given to Orion who finds it not easy being a leader.
At the time Sirius and Shion try to reason with Monsoon but they are making little progress but then discover the forest watchmen and their hunting dogs have entered Monsoon's territory. Despite trying to talk the dogs out of staying away from Monsoon two of them are killed but the akitas manage to save the third one. Gennai and his group begin the assault on Monsoon.
Monsoon manages to escape the dogs again and Sirius is still determined as ever to talk with Monsoon to end the fighting despite anyone's protest. Seeing no choice left Rigel and the others decide to inform the humans of Monsoon's existence.
Meanwhile, Gin informs Weed and the others of Orion and his siblings actions and Orion comes to visit them but is getting anxious. So Akame suggests that he trains and refines his skills with the sickle but after days of searching for it the weapon is no where to be found and Orion suspects that Sirius hid it. When Orion returns to Akame to report that sickle is missing the kishu suggest that Orion learns the secret techniques of the ninja dog.
While still in Monsoon's territory, Sirius and everyone hear scream and the two discover that Monsoon has killed a mother bear and was going to kill the cub too. Horrified by Monsoon's actions Sirius dives in to save the cub and after everyone escapes Monsoon, Rigel asks Sirius why is he doing this and all he says is that all this fighting is happening in the name of revenge and he doesn't want anymore lives lost. Sometime later, Sirius and Zion name the cub Chibi and teach him dog language.
Understanding that Chibi wasn't safe Sirius begged Rigel and Rocket to take him to Ohu and they agree meanwhile Orion continues his training. Once in Ohu the dogs considered Chibi an enemy until Rigel and Rocket explained everything but Orion was left unconvinced that he turned his anger on his friends until calmed down by Jerome and Kyoshiro. Meanwhile, Sirius continues his peace negotiations with Monsoon and has gained Gennai and the Kamakiri brother's support making Sirius' bond with the bear.
Sometime later, Akame revealed that he hid the sickle because the youngsters inexperience. He is alert by Orion's progress by Shirozaru. Unsai begins to show great concerns for the younger ones due to them losing their lives so easily and the army's influence on Orion as well as understanding that Monsoon only grew through hatred. As apart of his progress Sirius starts to learn the bear language but when he sees Gennai and the brothers go back on their promise the akita turned his fangs on them. As the fight continues on, Sirius departs from his family's pack attempting to make peace between the Ohu Army and the bears but the wars ends with not only Monsoon's demise but Sirius' as well.
After Sirius' and Monsoon's deaths, everyone returns to Ohu except for a grief stricken Orion who blames himself for his brother's death. Worried for him his friends look everywhere and while searching because of Monsoon's actions most of the dogs believed the Ohu Army perished, leading Japan into another power struggle this time with the Ak-ari troops.
As the fight begins can the Ohu army find Orion, get him out of his grief, stop this new threat and help Orion grow into the leader he was meant to be.