Ko-Hoki occupies the deepest stratum of the school, the chapter in which the Hoki name first enters the record at all. The published commentary places Yasutsuna roughly contemporary with Munechika in the late period, correcting the older reference-book dating that pushed him back to the Daido era (806 to 810), and the gap between that legend and the inferred reality is itself a measure of how far back this phase reaches. Gathered around Yasutsuna is the Ohara group: Masamori (read also as Sanemori), transmitted as his son, with Aritsuna, Sadatsuna, Yasuie and Masakage (Sanekage) named beside him, a circle the describes as flourishing from the late into the early period. Hoki Kunimune extends the manner toward the age, his work judged to stand apart from Saburo Kunimune and possibly to predate him. Signed survivals at this antiquity are thin, more frequent under Yasutsuna than any other hand, and the Dojigiri Yasutsuna fixes the period's highest reference.
The style of this phase is read first in the steel. The return blade after blade to with standing grain, mixed with and in large pattern, thick , entering and patchy that rises into or across a ground of darkish, blackish (kanai-iro) tone. The temper rests on a base broken into , with , and shallow mingled small in scale, the lying thick and the tending to or ; and run insistently, and many blades drop the edge above the in . The keeps an archaic form with and a small , the running straight to or . Against the later Sue-Hoki the contrast is one of vigor and individuality: where the late phase settles this rustic register into something tamer and more regular, Ko-Hoki stands open, dark and forceful, each hand still distinct, Yasutsuna's running larger while Sadatsuna's finest signed work brightens unusually toward .
For the recurring task is to separate Hoki from , whose contemporaneous it superficially resembles. The draw the line on close view: steel is bright and tight, while the Hoki stands open and dark with conspicuous large , a clouded , and and working through a softer provincial edge that carries an ancient fragrance (koko). A thread of Yamato spirit is read in certain pieces, such as the unsigned Yasutsuna long held in the Maeda house, and Sadatsuna's point toward even as judgment keeps him in Ko-Hoki. Yasutsuna and Masamori head the phase, and the signature habit of cutting (綱) larger than (安) and shifted to the right is a noted diagnostic. The Dojigiri stands above the named works, with the Imperial Yasunori and Sadatsuna and Kunimune blades transmitted through the Tokugawa, the Satake of Dewa, and the Hisamatsu-Matsudaira, the provenance running through the houses that held the country.