You can install SessionBox for Opera from the Chrome Web Store.

Step-by-step guide

1. To enable Chrome extensions in Opera, first you have to install the Download Chrome Extension from the Opera add-ons page:

2. After the extension installed, open chrome web store:
3. Click on the add Add to Opera button.

4. You can ignore the Compatibility notice, SessionBox is compatible with Opera. Then open Opera's extension page and look for the SessionBox extension. To finish the installation click on the install button.