Woo may be written with either of two hanja (禹 and 于).
Each has one bon-gwan: for the former, Danyang, North Chungcheong Province, and for the latter, Mokcheon-eup (목천읍), Dongnam-gu, Cheonan, South Chungcheong Province, both in what is today South Korea.
[1] The 2000 South Korean census found 180,141 people with these family names.
[2] In a study by the National Institute of the Korean Language based on 2007 application data for South Korean passports, it was found that 97.0% of people with this surname spelled it in Latin letters as Woo in their passports, while only 1.6% spelled it as Wu.
Rarer alternative spellings (the remaining 1.4%) included U and Wo.