Origins
Daur Origin
According to the page's citation of Heilongjiang Gazetteer and Chinese Names: The Daur, the Daur surname 鄂拉 may also be written 敖拉, meaning “mountain” in Chinese. It was later changed by some people to the single surnames 敖 or 单, using respectively a homophonic rendering of the initial sound or of the Chinese meaning “mountain.”
Reference: 鄂拉姓_鄂拉姓的来源和历史_百家姓
Historical Records
Manchu Attestation
The page, citing Wang Wanbang's Dictionary of Surnames, describes 鄂拉 as a Manchu surname and gives Ahabo, a Zhenghong Banner Imperial Guards commander during the Jiaqing era of the Qing, as an example. It also says that the Dictionary of Surnames records it on the basis of the Dictionary of Chinese Names and notes the possible pronunciation “āolā.”
Reference: 鄂拉姓_鄂拉姓的来源和历史_百家姓