Historically, Oichi was the younger sister of the Japanese warlord Nobunaga Oda. She was married to Nagamasa Azai to help foster good relations between the Oda and the Azai, but after relations turned sour, the Oda clan defeated the Azai and Oichi returned to the Oda. She later remarried, this time to the Oda retainer Katsuie Shibata. However, after Nobunaga's death, Shibata fought Hideyoshi Toyotomi for the right to succeed Nobunaga's command and lost at the Battle of Shizugatake. He retreated back to his home at Fukui Castle, where Oichi remained with him as Toyotomi's forces set the castle ablaze. It is believed that Oichi died in the fire.
Oichi appears as an NPC in Kessen III. Upon agreeing to the alliance with Nagamasa Azai, Nobunaga and his forces must provide an escort and protect Oichi's carriage from enemies while traveling to meet her betrothed. Later in the game, she tearfully returns to the Oda after Nobunaga defeats the Azai clan.
Samurai Warriors Series
In the original Samurai Warriors, Oichi is depicted as being a young teenager no more than fourteen or fifteen years old with a very playful personality. She fights using a kendama, a type of Japanese cup and ball toy, and her story mode sees her fight to both stop Nobunaga from destroying the Azai and save him from Mitsuhide Akechi's assassination plot.
In Samurai Warriors 2, Oichi's character design was changed greatly. Redesigned as an adult, she has a much calmer, more mature personality, and when the Oda and Azai fight, she sides with Nagamasa instead of making any attempt to foster peace as she had in the original game. However, she still fights using a kendama.
In Samurai Warriors 3, Oichi received another major change to her design; her kendama was replaced with bladed hoops. Personality-wise, she is very much like her appearance in Samurai Warriors 2, and after the Oda and Azai go to war, she sides with her husband over her brother. The story modes of other characters in the game depict the latter point in her life when she has remarried to Katsuie Shibata.
In Pokémon Conquest, Oichi appears early on quickly befriends the protagonist. She fights to help the player take control of the Ransei region and thwart Nobunaga's ambition. Her appearance is based on that of her Samurai Warriors 3 incarnation, and she is partnered with a Jigglypuff when she joins.
Here she initially goes by the name of Oyu, so as to hide her identity as the sister of Nobunaga Oda, who in the Onimusha series is an evil demonic warlord bent on ruling the world. Though she will eventually reveal her true identity to the main character Jubei Yagyu, whom she grows feelings for. Before Jubei finally reaches Nobunaga and engages in battle with him, he tells Oyu to run away so as to avoid having to fight her brother.
Her appearance in Onimusha 2 differs greatly from her appearances in the Samurai Warriors franchise, as she is dressed in armor and fights with a sword and round shield, as shown to the right.