If Opera Presto (Opera 12.17 and older) copy bookmarks.adr from the old Opera's profile folder to the new one. If Opera Blink (Opera 15 and newer) copy bookmarks* .* from the old profile folder to the new one.
On Opera Blink, don't forget to enable the bookmarks bar on settings.
If you had Opera Presto and now have Opera Blink, you'll have to reinstall Opera Presto again to be able to import your bookmarks.