In Chrome you have chrome://bookmarks/?id=2 for example to directly load "other bookmarks" by URL. That doesn't seem to work with 'opera://bookmarks/?id=2. But, if it did, there could at least be an extension that adds an icon to the sidebar where when you click it it goes to "other bookmarks".
Speaking of extensions though, you ought to see what's available in https://addons.opera.com/en/search/?query=bookmarks. There might be a bookmark sidebar extension that lists things how you like.