I wish Opera had the feature that Firefox does of being able to restore recently closed tabs and windows.
For now, there are a few things you can do:
Install Session Buddy to keep track of your current session and to save a session for later.
Right-click on a tab and choose "Save all tabs as Speed Dial folder". Then, if you ever have to, right-click the folder and choose "Open all in tabs". You can also goto the URL opera://bookmarks and move the folder out of the start page into "other bookmarks" or the bookmarks bar where you can right-clck the folder and choose to "open all" there too.
You can "Save all tabs as a Speed Dial folder" often and delete the folder to kind of keep a backup of your opened tabs. You can even right-click the folder and name it "Opened Tabs". And, if you open a new tab you want to keep, click the heart icon in the address field when on the page and choose to bookmark it to that speed dial folder.