Ou is the pinyin romanization of the Chinese surnames 區 and 歐, which share a common origin with the compound surname Ouyang (歐陽), from the ruling family of the State of Yue during the Spring and Autumn period. They are commonly romanized as Au or Ao in Cantonese.

Ou 歐 is listed 361st in the Song dynasty classic text Hundred Family Surnames.^[1] As of 2008, Ou 欧 is the 134th most common surname in China, shared by 1.13 million people.^[2]

Most recently, by using the 2010 China census data and statistical analysis data that included random sampling from Taiwan, Hong Kong and Macau, the Fuxi Culture Research Association ranked the surname 区/區 291st most common in China, shared by around 199,000 people (0.015% of the Chinese population) with the largest concentration of holders in Guangdong province.^[3]

Ou was the 27,293th most common surname in the United States during the 1990 census and the 11,845th most common surname during the 2000 census. Au ranked 11,417th and 5,195th, and Ao ranked 88,459th and 58,402nd.^[4]

During the 2010 census, Ou, Au and Ao ranked as the 7,891st, 4,919th and 41,501st most common surname respectively.^[5]

The Ou (歐) and Ouyang are considered to both descend from the ruling family of the state of Yue during the Spring and Autumn period.^{[citation needed]} With the 區 variant being descendants of Ou An (歐安), a wealthy and charitable merchant who came to the attention of Emperor Jing of Han, who ordered that Ou An do without the radical 'qian' (欠) from 歐, a character that means 'to owe'.^{[citation needed]}

Many Ou^[which?] have their ancestral roots in the Longgang, Zhongshan, Shunde, Panyu, and Xinhui areas of Guangdong.^{[citation needed]}

• O, for some Korean and Japanese surnames occasionally romanized "Ou"