Ordinarily, installing the new Opera versions (>15) doesn't do anything to the old Opera versions (12.16 or lower)... the new versions should install separately into their own subfolders, and one can actually run them side by side. Perhaps the shortcuts to Old Opera have been dumped, but you still should be able to open the Old Opera version directly by clicking on the opera.exe file in Old Opera's installation folder to start it... if that works as it should, then simply create a new shortcut to that file to use it thereafter.

Assuming the Old Opera version is still intact, you could export your original bookmarks from it via Menu > File > Import and Export > Export as HTML... > choose a file name and place to save it > Save. Most browsers can import from an html bookmarks file, including the Firefox 26 version I'm using at the moment.