With Opera 26, in Windows (possibly on some earlier versions too):
Have whatever webpages wished showing on TabsBar
Right-click on empty space of TabsBar
Option shows: "Save Tabs as Speed Dial FOLDER", ... go ahead and do that.
=> This will create folder onto SpeedDial
=> Thereafter, RIGHT-clicking on said saved folder, will present "Open All" <=.
-You can rename that folder once created.
(If one just CLICKS on said folder, folder opens, and an indiviual webpage of that folder can be opened).