There are several uses for this interjection:
1. When people dance traditional dances, people around can support and cheer on them by saying 'opa!'. It has then a meaning of joy and excitement.
2. When we want to say 'stop', 'hold on'. It has then an eaning of irritation.
3. When we want to emphasise
4. When we want to express surprise
5. When we want to defuse a stressing situation
2."Opa my friend, you're talking so long let me say something!"
3. "[Somebody is helping you in your parallel parking] Go on, go on, you have space... a little more, again a little... Opa, you're going to touch the car behind!"
4. "Opa, what are you doing here mate? I didn't know you were taking Greek classes!"
5. "[Somebody lets a plate drop and it breaks] Opa! Were you dreaming or what? You want another, ahah?"