Tsohuwar Cocin Slavonic (OCS) tana nufin harshen adabin Slavic na farko da aka sani, wanda Slavs Orthodox suka yi amfani da shi da farko don ayyukan liturgical da na addini. Ya dogara ne a kan yarukan da ake magana da su a ƙarni na 9 a yankin Tasalonika, a ƙasar Girka a yanzu, kuma ’yan’uwa Cyril da Methodius ne suka halicce shi, waɗanda ake ɗauka manzannin Slavs ne. OCS shine yaren Slavic na farko da ya sami rubutaccen haruffa, wanda aka sani da Glagolitic, wanda daga baya aka maye gurbinsu da haruffan Cyrillic. OCS ya taka muhimmiyar rawa wajen bunkasa harsuna da al'adun Slavic, kuma ana iya ganin tasirinsa a yau a cikin harsunan Slavic na zamani, musamman a cikin kalmomin addini.