Europe
歐 is an older form used for Europe, now largely replaced by 欧 in modern Japanese. Its standalone meaning is tied to that geographical reference.
歐 is the traditional form of 欧. It combines 區 (a region or area) with 欠 (a person with an open mouth, often related to actions or sounds). The character was used phonetically to represent the sound 'ō' in the name Europe.
The left side 區 looks like a map of a region with boxes for different areas, and the right side 欠 is a person. Together they form the old character for Europe, a region of people.
For オウ, imagine a person from Europe saying 'Oh!' (オウ) when they see the map-like 區 on the left.