You can open a window and open just those sites. Then, you can right-click on one of the tabs, goto "save" and choose "all tabs as a speed dial folder". Then, on your start page (when you open a new tab), you can right-click on that folder and choose "open all in tabs". You can do the "open all in tabs" on that folder when you're in a new window or the existing window. You can choose to open all the tabs in the folder in a workspace in the sidebar. You can even leave those tabs in a workspace in the sidebar (of your main Opera window for example) so that they're always there. Just switch to that workspace when you want those tabs. Just note that if you have multiple windows open and want them all restored when you open Opera back up, make sure the startup setting at the URL opera://settings/onStartup is set to Retain tabs from previous session and that you close Opera by going to Menu -> Exit without closing any windows via the X buttons.
Now, if you actually need a separate Opera profile for separate session data etc., you can just create a shortcut that creates/opens a different profile. Then, in that profile, at the URL opera://settings/onStartup, you can set that profile to open the specific set of pages you want. Could be easier to set up, but that's a workaround for now.