Osumi no Jo Fujiwara Masahiro was a native of Obi in Nisshu (Hyuga Province) who rose to prominence within the Horikawa school during the Momoyama and early Edo periods. He is said to have been either a nephew or a disciple of Horikawa Kunihiro, though the precise nature of the relationship remains uncertain. The NBTHK consistently notes that three extant works by Masahiro — one each of katana, wakizashi, and tanto — bear the date "Keicho 11, third month, an auspicious day" (1606), and that judging from their workmanship they already display considerable maturity. That he had received the court title Osumi no Jo by this early date indicates senior standing within the group, second only to Arikuni's receipt of the title Awa no Kami in Keicho 2. The Yako Meishushi appraises him directly: "It is said that he served as Kunihiro's substitute; he is extremely skillful." After Kunihiro's death in Keicho 19 (1614), Masahiro is thought to have returned to his home province of Hyuga, where he continued forging under signatures reading "Nisshu ju" or "Nisshu Obi ju."

Masahiro's forging presents the standing-grain itame-hada mixed with mokume that yields the distinctive, roughly textured jigane — described repeatedly by the NBTHK as zanguri — characteristic of Horikawa works. Particularly conspicuous intermixture of mokume is recognized as a distinguishing personal trait of this smith. His hamon is typically a comparatively calm ko-notare mixed with gunome, ko-gunome, and togariba; one does not encounter large, flamboyant o-midare. The nie adheres well, sunagashi and kinsuji appear throughout, and the nioiguchi shows a shizumi — subdued — tendency that the NBTHK identifies as a hallmark shared with Kunihiro's own range. Additional features linking master and student include yakikomi below the machi, hotsure along the habuchi, and occasional small yubashiri that impart an antique flavor. His manner of workmanship, signature style, yasurime, and overall nakago construction are consistently described as the closest in resemblance to Kunihiro among all members of the school, reinforcing the understanding that he frequently served as one of Kunihiro's daisaku-sha.

Within the Horikawa circle, Masahiro occupies a position of particular scholarly interest precisely because his surviving output is extremely small, yet the works that remain demonstrate mastery across multiple forms — katana, wakizashi, tanto, and the rare naginata. Many of his katana are of exceptional length, and the NBTHK repeatedly observes that despite this great length, his blades show no imbalance or breakdown of execution. Several setsumei note that certain works appear to aim at the manner of the superior Soshu masters, and one wakizashi in particular is identified as targeting the style of Sadamune. His tangs are notably long in proportion, and some are thought to have been deliberately fashioned to evoke the appearance of greatly shortened older blades — an utsushi sensibility that lends his work an especially antique character. The recurring evaluative phrase across the designation records is that his finest pieces stand "among the superior examples by this smith," a measured but authoritative endorsement that places Masahiro as the closest and most capable associate within the Horikawa lineage.