Can i play online games in EU which were bought from the us? please help

So i got an overwatch disc from the us cuz it was cheaper than germany, and for some reason the worker told us that "online will not work outside the us" i didnt really understand what she meant, so i thought that some reddit users could help me. please help cuz i already bought the game and i wanna know if i need to return it or not ASAP.