Signed works by Nagamitsu survive in greater numbers than those of any other smith of the Kamakura period: the published sources open his record with the statement that his "extant signed works are the most numerous among Kamakura-period swordsmiths"[[c:1]], and that every one of them shows "no unevenness in the making"[[c:2]]. He was the son of Mitsutada, founder of the Osafune school, its second generation, and "a master ranking alongside his father"[[c:3]] in the NBTHK's words. The record gathered here holds 173 signed blades against 63 unsigned; "the two-character form is the most numerous" of his signatures[[c:4]], and dated pieces such as an ubu tachi of Einin 2 (1294) fix his chronology as few contemporaries allow.
His style divides broadly in two, a formula the published sources repeat across decades of designations. One manner is a robust construction carrying on the feeling of his father Mitsutada, tempered in a flamboyant midare dominated by choji; the other is a form of standard or slender width in a comparatively calm suguha-toned temper with choji-ashi. In the exuberant manner the hamon is a choji-midare mixed with gunome, and ashi and yo enter in abundance. Nioi predominates with ko-nie, fine kinsuji and sunagashi run through, and the nioiguchi is bright. The element the sources single out as his own is the "round-topped, plump choji"[[c:5]], a full swelling crest not shown in his father's record. The boshi is ko-maru far more often than any other form, the published sources reading again and again "shallow notare, returning short in ko-maru at the point"[[c:6]]; on the more active blades it runs in as midare-komi before that turn-back[[c:7]], and on a minority it stops in yakizume[[c:8]], the whole settling toward what the notes call the atmosphere of the so-called Sansaku boshi[[c:9]].
The jigane is an itame that tends in places toward a slightly standing grain and on many blades tightens into a refined ko-itame. Very fine ji-nie adheres thickly, fine chikei enter, and a vivid midare-utsuri rises clearly, present on the great majority of his blades. Of one signed late tachi the published sources write that its tightly compacted itame, with extremely fine ji-nie and minute chikei, is a forging "combining precision with beauty"[[c:10]]; the same notes return again and again to steel that is bright and clear.
Three registers can be followed through the record. The earliest lies closest to his father, a choji-dominant temper into which kawazuko mingles, work that "truly calls Mitsutada to mind"[[c:11]]. The prime works carry the rounded plump choji with gunome, the vivid utsuri and the Sansaku boshi. The calm suguha manner gathers late, around blades cut with the title Sakon Shogen and with long signatures. Transmission texts of the Muromachi and Edo periods held that there were two generations, the Shogen pieces belonging to a second; the NBTHK observes that "no difference between a first and second generation can be found in the signature characters"[[c:12]], and the persuasive reading now takes Shogen Nagamitsu as the work of his later to final years. In the tanto of this late register appear the first buds of kataochi-gunome, the form his son Kagemitsu would complete.
His flamboyant works can at first glance resemble the Ichimonji school, and the published sources draw the line by his own features: the gunome stands out more prominently within his midare, the pattern grows calmer above the monouchi where the yakiba drops distinctly lower, and the boshi settles toward the Sansaku form. Against his father he is told by the rounded swelling crests and by a temper that keeps its composure where the early works flare. His late suguha and the kataochi buds of his tanto open directly onto Kagemitsu, so that Mitsutada, Nagamitsu and Kagemitsu form the spine of the Osafune mainline, with Nagamitsu its broad and steady center.
Fujishiro ranks him Sai-jo saku. Twenty-six of his blades are Important Cultural Properties, the most of any swordsmith, and six are National Treasures; beneath them stand twenty-eight Tokubetsu Juyo and one hundred and forty-four Juyo, out of two hundred and fifty-three designated works on record. The meibutsu Daihannya Nagamitsu passed from Ashikaga Yoshiteru through Miyoshi Nagayoshi to Oda Nobunaga and Tokugawa Ieyasu, and the wider denrai names Akechi Mitsuhide, Toyotomi Hideyoshi, the Asano of Geishu, the Hosokawa of Higo, the Bizen Ikeda, the Uesugi, the Maeda, the Mito, Owari and Kishu Tokugawa houses, and the Imperial Family. His National Treasures and Important Cultural Properties are patrimony, preserved among holders that include the Tokyo National Museum, the Kyoto National Museum, the Tokugawa Art Museum, Atsuta Jingu and Itsukushima Jinja. Because he signed so freely, the body of work in the Tokubetsu Juyo and Juyo tiers is substantial, and a signed Nagamitsu, though it comes to market only rarely and at the very top of it, is not wholly beyond a patient collector's reach in the way the great unsigned masters are; when one appears, it is an event.
