The 1590 Siege of Oshi (忍城の戦い, Oshi-jō no tatakai) was one of many battles in Toyotomi Hideyoshi's campaigns against the Hōjō clan during Japan's Sengoku period.

The castle was built on a small elevation near the Tone River and used surrounding marshes and swamplands as part of its outer defenses.

He had left his home castle defended by only 619 samurai and 2000 local conscripts led by his daughter Kaihime and younger brother Narita Nagachika.

This included a copy-cat effort to flood the defenders using the same method that Hideyoshi used at his famous Siege of Takamatsu.

It subsequently affected his ability to gain the loyalty and support of Japan's other powerful daimyo after the death of Toyotomi Hideyoshi.