I had the same question as per this person here:
Re: shortcut for bookmark folder on bookmarks bar
And then I discovered the answer after some "playing" with the Opera browser.
Firstly, I can only guest this is the "bookmark folder on bookmarks bar" as mentioned by the OP otherwise my answer won't work..
STEPS:
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Open Bookmark page and click Bookmark Bar.
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Drag and drop the desired bookmark folder to the bookmark bar.
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DONE! The folder should appear now on the bookmark bar.
Good Luck!