Nagamoto is traditionally regarded as a disciple of Osafune Nagamitsu, and by some accounts his younger brother. He was active during the late Kamakura period, with dated works from the Shodan era (beginning 1299) confirming his period of production. Critically, there exist tachi bearing Nagamitsu's signature in which the manner of inscription — the meiburi — is demonstrably by the same hand as Nagamoto's own signed works. From this evidence it is inferred that Nagamoto served as one of the daisaku-sha (substitute smiths) working on behalf of Nagamitsu, the head of their lineage, and that he was among the principal representatives within the Osafune school. Extant signed works by him are exceedingly rare, a circumstance likely attributable to the proportion of his career spent producing blades under his master's name, and possibly to later-generation alterations of the character "moto" to "mitsu."

Nagamoto's style is characterized by slender and gentle tachi sugata with pronounced koshizori and funbari, often terminating in ko-kissaki. His jigane presents tightly forged ko-itame-hada, sometimes mixed with mokume, bearing fine ji-nie and abundant chikei, with midare-utsuri standing out prominently. His hamon typically centers on ko-gunome mixed with ko-choji and pointed elements, approaching Nagamitsu's calm manner; alternatively, he produces a quiet suguha into which choji-ashi are interspersed. The nioiguchi tends toward shimarigokoro — a characteristic tightness — and the boshi frequently becomes shallowly notare, turning back in ko-maru to form the pattern known as the sansaku-boshi. In works of broader temper, deep nioi with well-adhering ko-nie is accompanied by sunagashi and kinsuji, yielding a vivid jiba.

Within the Osafune lineage, Nagamoto occupies a distinctive position as one whose workmanship, while closely aligned with Nagamitsu's refined idiom, nonetheless displays an identifiable individual manner — a quieter, more restrained approach that distinguishes his hand from that of his master. The NBTHK has repeatedly affirmed that "the characteristic features of this smith are clearly expressed" in blades attributed to him, and has noted the particular documentary value of his dated works. As a smith of whom very few examples survive, each authenticated work carries heightened significance, and the consistent quality of extant pieces confirms Nagamoto as a capable and accomplished artisan of the first rank within the Nagamitsu circle.