Oio-jinja, Shinto shrine in Japan

Oio-jinja is a Shinto shrine in the Ino district of Sosa, Chiba Prefecture, listed in the Engishiki shikinaisha as a minor shrine of Sosa District, Shimousa Province. Its main deity is Asahiko-no-mikoto, son of Futsunushi-no-mikoto of Katori Jingu, with Iwatsutsuo-no-mikoto, Iwatsutsume-no-mikoto and Kuninotokotachi-no-mikoto also enshrined. A sacred cedar designated a city natural monument stands behind and to the left of the hall.

According to shrine tradition, Oio-jinja was founded in the 7th year of Emperor Sujin's reign. It appears in the Engishiki register as a minor shrine of Sosa District, Shimousa Province, and served as the district's sōja. The hall burned in 1369 and was rebuilt by the Chiba clan, under whose protection it remained until their decline; in October 1873 it was ranked a gōsha.

The shrine's name is read Oio, and its ritual officials were the Katori clan, who served Katori Jingū; Katori Ōi-shima, granted Jugoinoge for offering private grain to the Mutsu garrison, is said to have lived in Sassa District. Sassa Shrine, a subsidiary of Katori Jingū, was formerly maintained at the district's expense. The shrine was the sōsha of Sassa District.

Oio-jinja is about a 25-minute walk from JR Yōkkaichiba Station on the Sōbu Main Line. The shrine stands in the former Shimōsa province's Sōsa district, and a large sacred cedar designated a city natural monument grows behind and to the left of the main hall.

The shrine's sacred tree, a sugi designated a natural monument by the city, stands behind and to the left of the main hall. The grounds also held a sessha, Sazasa-jinja, whose rebuilding was once a duty of Sazasa district; the shrine's priests came from the Katori family.