The Ōtsuki school (Ōtsuki-ha) emerged in late Edo period Kyoto as a distinguished lineage of tōsōgu craftsmen specializing in pictorial metalwork. The school's founder, Kawarabayashi Hideoki, established a workshop tradition that emphasized rigorous technical training and mutual emulation among students. His most accomplished disciple, Matsuo Karoku (1815–1875)—who signed his works as Gassan, Kongō Gassan, Kongōsai, or Tōsōtei—carried forward the master's methods with exceptional fidelity while developing a distinctive voice characterized by bold composition and meticulous execution. Working alongside fellow students including Sasayama Atsuoki and Tenkōdō Hidekuni, these artisans formed a cohesive workshop culture that valued both technical precision and creative interpretation of classical themes.

The Ōtsuki technical repertoire centered on takabori (high relief) and iroe (polychrome metalwork) executed on diverse base metals including shakudō, iron, brass, and shibuichi. Characteristic works combined extremely fine nanako grounds with dramatically raised relief elements, creating powerful contrasts between surface and sculptural form. The school favored ambitious pictorial subjects drawn from military history, natural imagery, and classical literature, rendered with keen attention to compositional balance and spatial dynamics. Chiselwork displayed a distinctive efficiency and deftness, with subtle manipulation of ground planes to suggest atmospheric depth and movement. The integration of precious metal inlay—particularly gold employed in sweeping, vital forms against restrained iron grounds—became a hallmark of the lineage's aesthetic sensibility.

The Ōtsuki school's influence extended through the late Edo period until the early Meiji era, when traditional sword mounting declined with the abolition of the warrior class. Gassan's works, in particular, represent the culmination of the school's technical and artistic achievements, demonstrating how late Edo metalworkers synthesized inherited workshop disciplines with individual creative vision. The school's emphasis on pictorial narrative, sculptural boldness, and technical virtuosity established standards that distinguished Kyoto tōsōgu production in the final decades of traditional samurai culture.