Add --allow-outdated-plugins argument to an Opera launch shortcut like this:
"C:\Program Files\Opera\launcher.exe" --allow-outdated-plugins
Note that starting from version 30.0.1835.125 the browser seems to ignore this key and it blocks usage of outdated plugins anyway. Till there is no way to allow outdated plugins in the new versions, you could disable automatic updates and stay with version 30.0.1835.88 (or earlier).