Opera also has lots of options related with sessions, you can even store the current one to load it later. I do this when i make research in some matter, instead of making lots of bookmarks. Another advantage of storing sessions is that it stores the history, wich means you'll still be able to go back on the pages (at least, most of them).
Notice that the "restore crashed session" is default on opera. If opera crashes, in the next time it'll open with the tabs (and windows) it had before. The option kimmchii mentioned allows you to start with those same tabs the enxt time you start opera, even if it didn't crash.