The term "Ossho" means the lords of Shoen (manor in medieval Japan) were exempted from and allowed to collect Zatsueki (a kind tax under the Ritsuryo system) (sometimes inclusive of the portion of Kanmotsu (tribute goods -supposed to be paid to Kokuga (provincial government offices) as well as the land of such Shoen.
Many Ossho were the former territories of the Imperial Court or Imperial Family which became Shoen after being donated to temples etc. Zatsueki gained from such Shoen was originally collected by Kokuga together with Kanmotsu, but in the case of Ossho, it was directly delivered to the lords of Shoen. It is said that this name was originally used by funin, (people who actually cultivated Ossho). However, while the lords of such Shoen had the privilege to gain a pre-fixed profit even in the year of poor harvest, such pre-fixed profit was low, itto (about 18L) per tan (an area), and they didn't have shitaji-shihaiken (the right to rule the land). Under such circumstances, lords of Shoen strived for the establishment of ichien-shihai (complete rule over people and land of Shoen) in collusion with local myoshu (owner of the rice fields).