Replay Heroes Story with Oichi on the Green route, after first beating it once while following either the Red or Blue route. When you reach the Siege of Karasu Castle stage, follow Tenkai. Defeat him and you will unlock the Incident at Honno-ji stage, which you can now play through as any available character. Next, replay Magoichi Saika and fight Oichi, once she appears near the end. Proceed onward from there and you will eventually find and be able to fight Oda Nobunaga, who is unlocked once you defeat him.