Old church slavonic was created in Great Moravia
"The language was standardized for the mission of the two apostles to Great Moravia (the territory of today's western Slovakia and Czech Republic; see Glagolitic alphabet for details). For that purpose, Cyril and his brother Methodius started to translate religious literature to Old Church Slavonic, allegedly based on the Slavic dialects spoken in the hinterland of their hometown, Thessaloniki, in today's Greece.
As part of the preparation for the mission, in 862/863, the Glagolitic alphabet was created and the most important prayers and liturgical books, including the Aprakos Evangeliar (a Gospel Book lectionary containing only feast-day and Sunday readings), the Psalter, and Acts of the Apostles, were translated. (The Gospels were also translated early, but it is unclear whether Sts. Cyril or Methodius had a hand in this.)
The language and the alphabet were taught at the Great Moravian Academy (Slovak: Veľkomoravské učilište) and were used for government and religious documents and books between 863 and 885. The texts written during this phase contain characteristics of the Slavic vernaculars in Great Moravia.
In 885, the use of Old Church Slavonic in Great Moravia was prohibited by Pope Stephen V in favour of Latin.
Students of the two apostles who were expelled from Great Moravia in 886, including Clement of Ohrid and Saint Naum, brought the Glagolitic alphabet to the First Bulgarian Empire and were received and accepted officially by Boris I of Bulgaria. He established the two literary schools: the Preslav Literary School and the Ohrid Literary School."
It began to be used two decades after it was created