Syncing Opera works differently than in other browsers. Opera won't sync your bookmarks in a way that you have the same bookmarks on all your devices. Rather it keeps them separate for any device. But you get access to the any devices' bookmarks.
I think, this is quite handy, as I'm happy that my Speed Dials e.g. are not the same on my private and my work device. But by the bookmarks menu I can easily click the bookmarks from the other one. You can even edit the bookmarks on other devices at least from desktop.
Unfortunately this doesn't apply to the iOS version which has no sync feature except for "Flow".