* Korea (South Korea and North Korea): It is one of the most common surnames in Korea, often written as "고" in Hangul.
* Japan: It is a common surname in Japan, often written as "古" or "河" in kanji.
* China: It is a less common surname in China, often written as "柯" or "寇" in Chinese characters.
* Vietnam: It is a less common surname in Vietnam, often written as "Ko" in Vietnamese script.
Therefore, it's difficult to determine the nationality of someone with the last name "Ko" without more context. You would need additional information, such as their first name, place of birth, or language spoken, to narrow it down.