The Osafune school emerged as the dominant sword-making tradition of Bizen Province during the late Kamakura period, establishing a production center that would endure through the Nanbokuchō and into the Muromachi era. The school's foundations were laid by Nagamitsu and his students, including Sanetika, Shinchō (Sanenaga), and other craftsmen who worked as substitute smiths (daikusaku) within the master's workshop. By the Nanbokuchō period, the school had diversified into several distinct lineages: the line descending from Kanemitsu and his students (including Henkyō, traditionally regarded as Kanemitsu's younger brother or disciple); the Motoshige group (with associated smiths such as Shigema, Shigemane, and Shigesue, some of whom may represent first and second generations); and collateral branches including the Ōmiya lineage (traceable to Kunimori, with representatives such as Morikage) and the so-called ko-sori-mono—a collective term for late Nanbokuchō makers such as Hidemitsu, Tsunehiro, Nariye, and Jikō, whose technical accomplishments often fell short of the principal Osafune lines.

Osafune workmanship is characterized by a tightly forged itame-hada, frequently mixed with mokume and exhibiting a tendency toward nagare (flowing grain), with fine ji-nie densely applied and chikei appearing throughout the jigane. A hallmark feature is the presence of midare-utsuri or suguha-utsuri—bright reflection patterns that stand prominently in the forging structure. The hamon exhibits considerable diversity depending on lineage and period: early works by Nagamitsu-school smiths favor narrow suguha with shallow urumi and ko-chōji admixtures, while Nanbokuchō-period production shows broader patterns—chōji-ashi running in saka-ashi fashion (as in Shigema), notare mixed with gunome (characteristic of Henkyō and the Motoshige group), or small-patterned midare with angular elements. The nioiguchi frequently displays a tightened or slightly subdued (shizumi or kozumi) quality, with ko-nie attaching well and activities such as kinsuji, sunagashi, and occasional yubashiri adding visual interest. The bōshi typically enters in midare-komi and turns back with a pointed tendency, though some examples show ko-maru returns with vigorous hakikake.

The Osafune school's enduring influence lies in its combination of technical refinement and adaptable production models. From the aristocratic patronage evidenced by Imperial Collection (Gyobutsu) holdings—such as Shinchō's dated Kagen 3 (1305) blade—to the workshop systems that enabled substitute smiths to sustain output through periods of military upheaval, Osafune represented both the apex of Bizen craftsmanship and a pragmatic response to evolving demand. The school's late Nanbokuchō works, including those by title-bearing smiths such as Shūri-no-suke (Morimitsu/Moromasa) and the transitional ko-sori-mono group, document the gradual stylistic shifts from the robust forms of the fourteenth century toward the more slender proportions and quieter temperlines of the early Muromachi period. Dated examples ranging from Einin (1293–1299) through Ōei (1394–1428) provide invaluable chronological anchors for the study of Japanese sword evolution, while the diversity of extant attributions—from confident signatures to mumei works appraised through workmanship alone—attests to the breadth and complexity of Osafune production over nearly two centuries.