Here is how to clean your internet history in Opera. The process is rather simple, but I can see how people can over look on how to do this.
To View The History
Click Help and then About. In the About Opera window you’ll see the full path to where the cached files are stored. Browse to that file path to view cached files
Deleting or Cleaning the Internet History Files
You can delete your history by clicking the “File” menu>>Preferences>> “History”>> and click “Empty now” button.
In addition, you can check “Empty on exit” if they wish for this task to occur each time they exit the browser.