The founder of the Chikuzen Sa School is known as O-Sa. Although he signed his blades with just the name, Sa , his full name was Saemon Saburo Yasuyoshi. O-Sa has long been considered to be one of the ten famous students of Masamune. Judging from the dates of the few dated pieces left by O-Sa that were made around the Ryakuou Era, he was one of the last students of Masamune.

O-Sa is considered to have been the son of Jitsua from Chikuzen Province in Kyushu. His father's works were typical of the type produced in Kyushu during the Kamakura Era, i.e. of a rustic and unrefined type of craftsmanship that was passed down from Ryosai through Sairen. The early works of O-Sa show some of those characteristics but he soon adopted Soshu traits and developed his own distinctive shape and style.

Few dated examples of O-Sa remain today. There are three tanto which are dated Kenmu 5 (1338), Ryakuou 2 (1339) and Ryakuou 3 (1340). Of these the first two showed definite traits of the old Kyushu workmanship and are thought to be his earliest surviving examples. The last one showed definite Soshu workmanship. The existence of a National Treasure tanto by his son, Sa Yukihiro, which was made in almost the exact same style as his father helps to answer some of the questions regarding the changing styles of O-Sa. Since this tanto is dated Kanno Gan Nen (1350), it shows that sometime between 1340 and 1350 O-Sa mastered his famous O-Sa style of workmanship.

In addition to his son, Yukihiro, other famous students of the Chikuzen Sa School were, Yasuyoshi, Yoshisada, Yoshihiro, Kunihiro, Hiroyasu, and Sadayoshi, The Hirado Sa and Oishi Sa should be considered to be minor branches of the Sa School.

The following blade characteristics should be considered typical of both O-Sa and his son, Sa Yukihiro. There are only a very few differences which will be noted.

SUGATA: There is only one surviving example of a long sword by O-Sa. It is a Kokuho tachi named Kousetsu Samonji. Tokugawa Ieyasu wore this tachi. Many tanto of O-Sa survive today. Only a few examples of Sa Yukihiro exist. Most tanto are short appearing more in the style of the Kamakura Era than the Nanbokucho Era. This is probably due to the fact that O-Sa worked around the end of the Kamakura Era and the very beginning of the Nanbokucho Era. Most are about 7 sun (21 cm) with the longest around 8 sun (31 cm). Unlike Kamakura sugata, however, his had slight sori and a thinner kasane. Generally the fukura was straight with no roundness. They were almost always mitsu-mune, but an occasional iori-mune example can be found. The tanto of Sa Yukihide differed in that they tended to have slight uchizori.

JITETSU:The jitetsu of O-Sa will be a beautiful koi-tame with some small areas of mokume. The steel will have a bluish tinge that will contrast with the bright white yakiba. On some blades you will find a nenrin-hada (hada shaped like the annual rings of as tree). Shirake-utsuri and nie-utsuri can be found.

HAMON: Generally you will find notare mixed with gunome. One of the famous characteristics of O-Sa was that his hamon starts in a yakikomi style and widens as it goes up. This means that the hamon started slightly below the machi in a form called koshiba. The hamon will be nie based with thick nie over nioi. The nie-guchi will be bright and clear. There will be ashi that tend to be oblique, as well as many hataraki (kinsuji, chikei, inazuma, etc.). Sa Yukihiro's hamon will differ slightly in that his will slant in a reverse direction in the monouchi area

BOSHI: His boshi is referred to as Sa boshi. They will be almost pointed in a thrusting manner with a long kaeri extending down the mune. There will be a bright and crisp nioi-guchi on the kaeri.

HORIMONO:Bo-hi, soe-hi, and futasuji-hi can be found.

NAKAGO: The tip of his nakago will be ha-agari-kurijiri in shape. The file marks will be o-sujikai.

MEI: Generally O-Sa signed Sa on the omote and Chikushu (no) Jyu on the ura. Sa Yukihiro signed Chikushu (no) Jyu Yukihiro on the obverse and the nengo would go on the reverse.