Woo (우/于), romanized as U (pronounced as 'ooh') in Korean Revised-Romanization is an uncommon Korean surname, similar to Chinese Yu. This hanja usually used in some ancient texts, several names and proper nouns in Chinese Grammar. This surname is the 82nd surname mentioned in the Chinese Song Dynasty's Hundred Family Surnames.
It is the 38th most common surnames in Mainland China and 140th most common surnames in Korean Republic. The name is transliterated as Vuu in Vietnamese but is very rare in Vietnam. Note that even though the literary usage of the character 於 was simplified to 于 in simplified Chinese, the surname usages of 於 was unchanged. So both are recognized as distinct surnames in both simplified and traditional Chinese.
The Yu (于) surname originated from the surname Ji (姬). According to history records, after King Wu of Zhou (surname Ji) overthrew the Shang Dynasty, he created many dukes. He gave his second son Yu Shu (邘叔) the Yu kingdom (邘国, in present Qinyang in Henan), whose descendants then acquired the surname Yu (于). The Yu lived in Kaifeng and Chenliu areas in Henan for a long period of time, becoming prominent families there. After the Tang (618-907) and Song (960-1127) dynasties, Yu families started to move to Huiji (now in Shaoxing, Zhejiang Province, PROC).
Back in Korea, the number of Koreans who bear the surname of 于 are fifty-four times lesser than the Koreans who born with the surname of 禹 (based on King Yu the Great of Chinese Xia Dynasty). Coincidentally, these two Hanja Characters are pronounced as Woo (우) in Hangul. Thus, there are 3,264 people in 1,047 families who bear the surname of Woo (于), based on the 2000 Korean Republic Census - provided by Statistics Korea (통계청/統計廳/Tonggyecheong/KOSTAT). The majority of its clansmen are rooted from Mokcheon Woo Clan (목천 우씨/木川于氏), a clan which is originated from Mokcheon-eup, Cheonan Dongnam-gu, Southern Chungcheong Province.
Mokcheon Woo Clan was founded by Woo Bang-nyeong (우방녕/于邦寧), a Goryeo scholar who lived during the reign of King Injong Wang Hae (1109-1146), 17th King of Goryeo Dynasty. Based on the Joseon Dynasty's Augmented Survey of the Korean Geography (동국여지승람/東國輿地勝覽/Dongguk Yeoji Seungnam), Mokcheon Prefecture is an area controlled by Baekje Kingdom before King Taejo Wang Geon of Goryeo unified three Kingdoms into single-nation of Goryeo Dynasty. In this area, there are four surnames that correspond to the animals such as Woo (우/牛 - cow), Sang (상/象 - elephant), Don (돈/豚 - Pig) and Jang (장/獐 - deer). The latter descendants of these clans revamped their surnames into present-day Woo (우/宇, 禹 and 于), Sang (상/尙), Don (돈/頓) and Jang (장/張). These clans are rooted from Mokcheon-eup.