Etymology: Greek, thematic vowel of many nouns and adjectives in combination — used as a connective vowel originally to join word elements of Greek origin and now also to join word elements of Latin or other origin
-O-
Meaning of -O- in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012