Osumi no Jo Fujiwara Masahiro was a native of Obi in Nisshu ( Province) who rose to prominence within the school during the and early periods. He is said to have been either a nephew or a disciple of Kunihiro, though the precise nature of the relationship remains uncertain. The consistently notes that three extant works by Masahiro — one each of , , and — 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 , where he continued forging under signatures reading "Nisshu ju" or "Nisshu Obi ju."
Masahiro's forging presents the standing-grain mixed with that yields the distinctive, roughly textured — described repeatedly by the as — characteristic of works. Particularly conspicuous intermixture of is recognized as a distinguishing personal trait of this smith. His is typically a comparatively calm mixed with , , and ; one does not encounter large, flamboyant . The adheres well, and appear throughout, and the shows a — subdued — tendency that the identifies as a hallmark shared with Kunihiro's own range. Additional features linking master and student include below the , along the , and occasional small that impart an antique flavor. His manner of workmanship, signature style, , and overall 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 -sha.
Within the 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 — , , , and the rare . Many of his are of exceptional length, and the repeatedly observes that despite this great length, his blades show no imbalance or breakdown of execution. Several note that certain works appear to aim at the manner of the superior masters, and one 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 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 lineage.