Not sure of any fix, but you do not need to have too many bookmarks before Opera may fail to show them as you want to seem them. I've been using Opera v 36 in Windows Vista for a few months and the advantage is that it is fast; but the disadvantage is that the bookmarks system works very poorly, failing to show bookmarks in the bookmark bar, failing to allow for bookmark folder navigation. Failure to show a bookmarks side panel. What I have to do is open bookmark manager, find that bookmark wherever it is in searching every folder individually, and then MOVE it manually to the folder where I want it.
Like I said, otherwise, Opera on Vista is good because it is fast and has a minimalist user interface appearance like google chrome.