Stop Opera from auto updating
Try the following steps:


Completely uninstall all the versions of Opera from your Mac.
Reinstall your preferred version of Opera.
Disconnect from Internet connection.
Launch and Quit Opera.
Launch Terminal.
Type this code and hit enter.

defaults write com.operasoftware.Opera OPDisableAutoUpdate true

Reconnect Internet, launch Opera, and select About Opera.
The screen will display if auto update has been disabled. It should look something like this:

Version information

Version:    49.0.2725.64 - Update checker is disabled

Update stream:  Stable

System: Mac OS X 10.9.5 64-bit