This tips is for Opera version 7.50 or higher. Go to 'Help->About Opera' to find the location of your Preferences file (opera6.ini). Then close Opera and navigate to the folder where the opera6.ini file is located. You can open the file with any text editor such as Notepad. Find the section labeled [Hotlist Window] (or if there isn't such a section, add it at the end of the file). Within that section add this line:
Hotlist single click=0
Save the file. Opera will now use double-clicks to open bookmarks (as was the default in prior versions).