Because of the law of open syllables, the final-b was often dropped. It was retained when the next word began with a vowel or withl orr. When combined with a word that began withv, it was also retained, but thev was dropped.
Vasmer, Max (1964–1973), “о”, inOleg Trubachyov, transl.,Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress
