Nagasone Okimasa (長曾祢興正) studied under Nagasone Kotetsu and was later adopted by him, succeeding as the second generation in the Kotetsu line. Among dated works that survive, the earliest bears an inscription of Kanbun 13 (1673) and the latest Genroku 3 (1690). His technical ability is consistently described as "highly accomplished, second only to his master," and the NBTHK repeatedly observes that Kotetsu's oeuvre likely includes works made as Okimasa's daisaku -- substitute forgings executed on the master's behalf. After Kotetsu's death, Okimasa took the public role while fellow students Okihisa and Okinao appear to have "supported him behind the scenes," an arrangement directly inferred from the inscriptions on a rare collaborative work.

Okimasa's forging typically shows dense ko-itame-hada mixed with mokume, with thickly applied ji-nie and fine chikei, producing a steel of notable clarity. His signature hamon is the linked gunome forming a juzu-ba (rosary-bead) pattern inherited from Kotetsu's approach; however, "by comparison the nioi here is deeper and the nie thicker and more abundantly applied, producing a spirited and forceful manner of finish -- something that may be regarded as Okimasa's particular strength." The NBTHK consistently identifies several distinguishing traits that separate master from successor: a wider yakihaba, the conspicuous handling of paired gunome within the temper, a somewhat coarser-grained nie that in places produces patchy unevenness and a tendency toward basake, and vigorously applied sunagashi. His works also display a bright nioiguchi that is uniformly saeru in both ji and ha. Beyond his favored juzu-ba idiom, Okimasa's range includes shallow o-notare with vigorous gunome-ashi, suguha-based compositions with round-headed gunome, and even an unusual division of suguha in the lower half with gunome-midare above -- possibly a reproduction piece. His signed inscriptions "vary greatly," with the character for "masa" appearing in both cursive and formal kaisho script, and signatures reading "Kotetsu Okimasa" encountered from time to time.

The NBTHK positions Okimasa as a smith of the first rank within the shinto period, whose bold manner of making harmonizes forceful nie with bright, clear steel. His works are praised both for their intrinsic quality and their documentary value -- the Kanbun 13 dated katana is "the earliest" among dated examples, and pieces bearing the "Kotetsu Okimasa" inscription strengthen the impression that "he truly inherited the position of Kotetsu's second generation." Works from his earlier output, around Enpo 4, possess "an impressive power, as though it allows one to glimpse the smith's ambition to pursue individuality while still inheriting his master's style."