Often interpreted as inherited fromOld East Slavic*очьнь(*očĭnĭ) fromоко(oko,“eye”); the semantic shift might be "visibly > obviously > prominently > very". The non-productive ending is similar to adverbs likeвкривь(vkrivʹ),впрямь(vprjamʹ),встарь(vstarʹ).
Vasmer, Max (1964–1973), “очень”, inOleg Trubachyov, transl.,Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress
Alexander Lehrman (1987), “The Etymology of OЧEНЬ”, inRussian Linguistics, volume11, number 1, pages31-36