If you have a backup of your previous Session_random_characters_here and Tabs_random_characters_here files, there's a pretty easy way to trick Opera into loading them.

1. Open Opera, so that it creates new versions of those files, with new random characters at the end.
2. Close Opera.
3. Copy the full names of both of those new files.
4. Delete the new files.
5. Rename your backed-up files to their corresponding new names.
6. Move your newly renamed backup files into the Sessions folder, if you had them somewhere else.

If you open Opera after this, it should load the backups and recover your session and tabs (at least it worked for me).