Ouyang Nana (欧阳娜娜) has four Chinese characters in her full name. Splitting after the first character would produce Ou as the surname and Yang Nana as the given name. That is wrong: Ouyang (欧阳) is one compound surname.
China’s official rules for spelling personal names define a compound surname as a surname containing more than one Chinese character, usually two. Ouyang (欧阳) and Sima (司马) are the standard examples. A name still has two basic parts—surname and given name—even when the surname occupies two characters. Sima Guang (司马光) divides as Sima / Guang, not Si / Maguang.
Pinyin makes the boundary visible. The surname and given name are separated, while the characters inside a compound surname are joined. 欧阳娜娜 is therefore Ouyang Nana, and 司马光 is Sima Guang. Ouyang is not a middle name, nor is Yang part of the given name.
Put the whole compound surname in the surname field
On a form with Surname or Family name and Given name fields, enter Ouyang under surname and Nana under given name. A website may later display the given name first, producing Nana Ouyang. The display order has changed; the fields have not.
The number of Chinese characters cannot reveal the structure by itself. A four-character name may contain a two-character surname and two-character given name, or a one-character surname and three-character given name. The national spelling standard also distinguishes a compound surname from a double surname formed by placing two surnames together.
Passports, tickets, visas and bank records add a practical constraint: follow the identity document. If the document uses capitals, fixed spacing or a particular order, reproduce those fields rather than redesigning the spelling. Do not split Ouyang into Ou Yang unless the issuing document itself does so.
Common compound surnames
Official Chinese name-transliteration guidance lists compound surnames including Ouyang (欧阳), Sima (司马), Shangguan (上官), Zhuge (诸葛), Situ (司徒), Yuwen (宇文) and Gongsun (公孙). They should be recognised as complete surnames before the given name is identified.