Not much is known about the Oni warrior Sougen Osho (宗元和尚), only that he fought in the Oni War with Marcellus over a 100 years ago. He is first referenced in a note written by him explaining the existence of a Ogre's Gauntlet and journals written by Saimyou that are found by Samanosuke Akechi and Kaede in Onimusha: Warlords. More of his past is revealed in Onimusha: Blade Warriors.
According to the first two journals, Sougen and Saimyou followed a group of demons that were kidnapping people around town, which lead them to a cave. The two fought the demons but were ultimately unsuccessful, Sougen was seriously injured and asked Saimyou to bring his body to the entrance of the cave. There he performed his final ceremonial rights and offered his body as a human sacrifice and placed a seal to the entrance of the cave to prevent the demons from getting out. He gave Saimyou a scroll and a jewel and ordered him to leave.
Onimusha: Blade Warriors
Sougen is mention by Kaede to Keijiro Maeda at the Three-story Pagoda, he's later mention by Guildenstern when he's shouting at Marcellus in the Ice Chamber and Kaede finds more information on both Sougen and Marcellus, to which she tells Samanosuke at the Ancient Castle (Flame).
- The name "Ogres" is first seen in Sougen's note from Warlords, this is also seen in the select menu screens. However, for the rest of the series they would be called the "Oni".
- It's not until Blade Warriors that he's given the last name "Osho".