Ou is the pinyin romanization of the Chinese surnames 区/區 and 欧/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.
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 only 199 thousand people (0.015% of the Chinese population) with the largest concentration of holders in Guangdong province. 
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.
The Ou (欧/歐) and Ouyang are considered to both descend from the ruling family of the state of Yue during the Spring and Autumn period. With the 区/區 variant being descendants of Ou An (欧安), 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'. 
List of persons with the surname
- 百家姓 [Hundred Family Surnames] (in Chinese). Guoxue. Retrieved 2014-10-28.
- 中国最新300大姓排名（2008） [300 most common surnames in China (2008)] (in Chinese). Taiwan.cn. 2009-01-06. Retrieved 2014-10-28.
- “百家姓”排行榜刷新“王”取代“李”成第一大姓氏，新华网，Retrieved 15 April 2013.
- US Census Bureau. Op. cit. Public Broadcasting Service. "How Popular Is Your Last Name?" Accessed 6 Apr 2012.
- Frequently Occurring Surnames from the 2010 Census. ""
- 欧安 Retrieved 3 April 2017.
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