Ō-yoroi was the Heian-period samurai armor style designed primarily for mounted archery.
The name means “great armor,” and the form developed between the tenth and twelfth centuries as elite warriors fought from horseback. Its large box-like shoulder guards, called sode, protected the upper body while still allowing an archer to draw a bow.
Ō-yoroi was constructed from small lacquered plates laced together with silk or leather. The armor included a cuirass, skirt-like kusazuri, helmet, face protection, and large shoulder defenses. It was expensive and associated especially with high-ranking mounted warriors.
The armor was less practical after warfare increasingly involved foot soldiers, firearms, and close combat. Later Japanese armor, including tōsei gusoku, used more compact metal components and provided greater mobility. Ō-yoroi is therefore best understood as the classic mounted-warrior armor of medieval Japan, not the standard equipment of every samurai.