Yasumitsu of worked in Province in the early period, around the Ōei era (1394–1428), the years in which a flourishing group of smiths revived after the slump of . The published sources call that group Ōei-, and they place Yasumitsu, with Morimitsu, at its head: "Yasumitsu, together with Morimitsu, stands as the twin pillars among the smiths of the early period collectively termed Ōei-." Beside the two leading hands they name Iesuke, Tsuneie and Sanemitsu as lesser masters of the circle. The school's ideal, the sources say, was a return to the period, an aim visible in its revival of the shape and of the that had fallen out of use under . He carries on the line of Nagamitsu and Kagemitsu, and his dated work runs densely through the Ōei years, the long signature Bishū Yasumitsu cut toward the with the date on the reverse.
His prime and most characteristic hand is a flamboyant one. Over an mixed with , the grain standing, the temper rises into a large-pattern built on whose base opens wide at the waist, the form the published sources treat as the school's own. and enter abundantly; the line is -based with , slight and fine playing through, scattering at times, the bright and clear. The enters and turns back to a pointed, tongue-like tip, with brushed through it. This turnback is his signature feature and the school's: the sources single out "a whose tip becomes pointed, the characteristic form generally called the rōsoku-no- (candle-wick)." It is the trait by which Ōei- separates itself from the revival its and otherwise evoke.
The is the throughout his work and is itself a point: a standing thickly mixed with , sometimes flowing, fine adhering like dust and -like dark lines entering, what the sources render as - no . Across it stands a reflection, and the sources distinguish two kinds: a straight or , and a that they read as the more archaic, recalling late- . Of one the published commentary remarks that its is "not the often seen in this period, but instead shows ." It is the standing, -bearing , not the smoother , that the judges use to fix the attribution. A , often with a paired , is finished round above the , a school tell, and the may carry , a sankozuka-, or carved deity names.
Alongside the flamboyant the sources mark a second, calm hand. It is a chū- or , the tight, bright and clear, faintly broken with and and frayed along the edge with and , the running straight to a . The judges note this hand is comparatively common in him, and more his than his fellow master's: "the , it would seem, is found rather more in Yasumitsu than in Morimitsu." His signing follows the registers that scholarship uses to sort the generations. The corpus is overwhelmingly and long-signed with an Ōei date; two-character are comparatively rare and tend to carry no date. Dated work survives from Ōei, Shōchō, Eikyō and Kakitsu, and the common reading takes the pieces dated after Shōchō as a second hand. The reference works place the Yasumitsu who styled himself Uemon-no-jō as the first generation, active in the Ōei years, count five generations of the name down to the end of , and assign a Sakyō-no- working in the Eikyō and Bunan eras, the so-called Eikyō-, to the next, whose temper is smaller in pattern and judged a touch inferior; the strict division between the generations the sources leave open for further study.
Within the Ōei- pair the two masters share the school manner, and the working distinction a collector draws between them is a fine one the sources state in their own words. Of a whose teeth point at the head the commentary observes that this is "a generally found rather more in Yasumitsu than in Morimitsu," and the is said of the calm . His can deceive: at a glance it recalls late- , and where the sinks and enter it can evoke , the sources writing of one such blade that "it has tempered a of tight , so as to suggest , yet this kind of workmanship is seen from time to time in Yasumitsu." The attribution settles back on him by the standing - and the Ōei . The religious carving he cuts at the , , the sankozuka-, names such as , is not his invention but an inheritance: "this is the carving tradition seen in work since Nagamitsu and Kagemitsu, and it is carried on down to ."
Fujishiro grades him Jō-jō , and his work carries weight in the designation record: four of his blades are Important Cultural Properties and four are , with forty-three beneath them, forty-seven in the and tiers together. He is Jō-jō and almost entirely signed, fifty-five signed pieces against none unsigned in the official record, and he left accomplished work in every form: , , , , and even a rare -signed great spear, the published sources noting that signed Ōei- spears are extremely scarce. His blades carry a distinguished provenance, twelve recording a history through the Yamauchi lords of Tosa, the Nabeshima, Maeda, Tōdō, Naitō and Akimoto houses, the Imperial Family, and the rōnin Horibe Yasubei (Taketsune) of the Akō affair. The Important Cultural Properties are held as patrimony and do not trade. Of recorded whereabouts his blades sit largely in shrines and museums, among them Atsuta Jingū, Jingū, Nikkō Tōshōgū, the Yūtoku, Hayashibara and sword museums, and the British Museum. A Yasumitsu in the or tier is not wholly beyond reach, for the corpus is large for so prized a name, but the finest survive in held collections and one comes to open hands only from time to time, a landmark of early- when it does.