Ōshima

Ōshima is the largest of Yamatai's main islands and the largest island in the Naichi archipelago. It is located to the north-west of Harukuni, and is divided from Harukuni by the Aki Strait. As of 2016, it has a population of roughly 16 million, a large portion of which are concentrated in the southern part of the island, in the South Ōshima Metropolitan Area.

Ōshima is typically divided into two areas, South Ōshima and North Ōshima. South Ōshima includes the provinces of Iwaki, Kuromori, Mizuho, Shinano, Yamagata, and Yamashiro. North Ōshima is comprised of the provinces Aomori, Masaki, Morioka, the Northern Islands, Sadogashima, Shiba, and Toyama. Administratively, the Northern Islands are also considered a part of Ōshima.

The majority of the population lives on the coastlines and in the plains. Historically, Ōshima has been considered to be the breadbasket of Yamatai, with large tracts of farmlands supplying the rest of the nation. In particular, the largely flat Mizuho Plain in southern Ōshima accounts for 32% of food production in Yamatai, while the smaller Shiba Plain in the north accounts for 10% of national food production.

South Ōshima is a rapidly developing new economic zone, and has historically been a heartland for industry. Stretching across Yamashiro and Iwate provinces, the South Ōshima Metropolitan Area is the third-largest metropolitan area in Yamatai, and is an economic and trade hub important in cross-Varunan trade. North Ōshima is largely rural compared to other parts of Yamatai, and primarily relies on agriculture and tourism for income. Studies have shown that an income gap between the north and south does exist across the "Hotaruyama Line", referring to Mount Hotaru, which lies on the informal divide between North and South Ōshima, though government efforts to develop and revitalise North Ōshima have long been underway.