Leechblock for Opera - Time management
"StayFocusd" seems to be the extension you need.
I've requested the port to Opera catalog which you can upvote: https://getsatisfaction.com/stayfocusd/topics/a-version-for-opera-browser
Meanwhile, you can download the .crx file from Chrome Store and drag'n'drop it to the Opera.
Or, alternatively, use this extension to install it directly from Chrome Store: https://addons.opera.com/en/extensions/details/download-chrome-extension-9/