三方原之戰
Starting around the time of the Battle of Mikatagahara (1573), perhaps in its aftermath, Ieyasu began to confide in Oai and sought her counsel on various matters. It may have been during this period that the two commenced an amorous relationship. Oai is credited with advising Ieyasu as the Battle of Nagashino (1575) approached, a major turning point in both Ieyasu's career and the history of Japan. It is also thought that Ieyasu continued to seek her advice concerning other battles and alliances, even as late as the Komaki-Nagakute Campaign (1584).