Greek surnames were widely in use by the 9th century supplanting the ancient tradition of using the father’s name, however Greek surnames are most commonly patronymics. Commonly, Greek male surnames end in -s, which is the common ending for Greek masculine proper nouns in the nominative case. Exceptionally, some end in -ou, indicating the genitive case of this proper noun for patronymic reasons. Although surnames in mainland Greece are static today, dynamic and changing patronymic usage survives in middle names where the genitive of father's first name is commonly the middle name (this usage having been passed on to the Russians). In Cyprus, by contrast, surnames follow the ancient tradition of being given according to the father’s name. Finally, in addition to Greek-derived surnames many have Latin, Turkish and Italian origin.
What does it mean to have the letters OU added to the ending of a males last name ?
some end in -ou, indicating the genitive case of this proper noun for patronymic reasons.