Old Church Slavonic

Old Church Slavonic (OCS) or Old Slavonic ( ) is the first Slavicliterary language and the oldest extant written Slavonic language attested in literary sources. It belongs to the Eastern South Slavic subgroup of the South Slavic branch of the Slavic language family and is the ancestor of Church Slavonic, which remains the liturgical language of many Christian Orthodox churches.

Historians credit the 9th-century Byzantine missionaries Saints Cyril and Methodius with standardizing the language and undertaking the task of translating the Gospels and necessary liturgical books into it as part of the Christianization of the Slavs. It is thought to have been based primarily on the dialect of the 9th-century Slavs living in the Byzantinetheme of Thessalonica. The oldest manuscript is usually thought to be the Kiev Missal.

Old Church Slavonic became the official language of the First Bulgarian Empire and played an important role in the history of the Slavic languages serving as a basis and model for later Church Slavonic traditions. Some Eastern Orthodox and Eastern Catholic churches use these Church Slavonic recensions as a liturgical language to this day.

As the oldest attested Slavic language, Old Church Slavonic provides important evidence for the features of Proto-Slavic, the reconstructed common ancestor of all Slavic languages.