NOTE: I also tried this plugin : https://addons.opera.com/en/extensions/details/bookmarks-manager/?display=en - yet when i export in Netscape format (HTML) it gives a (null) result for some reason.
It's likely there's a bug in its exportAsNetscapeBookmarksFile function. Or, the bug is in the recursive function that calls it.
The add-on looks to use Opera's bookmarks API to get the bookmark data to export. So, it's possible the bug is there even.
Either way, for the developer to trouble should it, he's going to need your bookmarks file in your profile folder so he can put it in his Opera profile and test to see what's wrong.
Another way it could be done is just have an add-on that loads the bookmarks file (json format) and parse its JSON (via JSON.parse()) into an object. Then, it'd just sift through the object look for the parts that are needed to output the bookmark data as HTML.
Either way, you have to find the developer or somebody that wants to do those things.
If you only need to do this one, even if you have a lot of bookmarks, you could just look at the bookmarks file in a text editor and do it by hand. Sure, it'll take you a while. But, so might writing/fixing an extension.
So, not really much help, but...