When the installer launches, you can click "options". and choose how you want to install it. The default option is "current user" where it installs to "%LOCALAPPDATA%\Programs\Opera" by default, which doesn't need admin privileges. If you choose "All users", it'll install to "%ProgramFiles%\Opera" by default, which would need admin privileges, and it should ask. Is it not in that case?
There are also command-line arguments for the installer if you're managing Opera installations.