This is a list of surnames in which the usage is Japanese; and the first letter is O; and the gender is unisex.
Ogawa 小川, おがわ JapaneseFrom Japanese
小 (o) meaning "small" and
川 (kawa) meaning "river, stream".
Okabe 岡部, おかべ JapaneseFrom Japanese
岡 (oka) meaning "hill, ridge" and
部 (be) meaning "part, section".
Okamoto 岡本, おかもと JapaneseFrom Japanese
岡 (oka) meaning "hill, ridge" and
本 (moto) meaning "base, root, origin".
Ono 小野, おの JapaneseFrom Japanese
小 (o) meaning "small" and
野 (no) meaning "field, wilderness".
Ōshiro 大城, おおしろ JapaneseFrom Japanese
大 (ō) meaning "big, great" and
城 (shiro) meaning "castle". It is especially common on Okinawa.