Scroll down any OSNews page, and click to the Opera button with your Opera browser. The OSNews sidebar panel will appear in your "Hotlist" bar (as long you have that bar visible that is).
The panel page is set to automatically reload itself every 1200 seconds. However, you might have to correct the Cache settings on your Opera preferences, to tell it to not use disk cache and to check "always" for Documents, Images and Other (the default cache scheme on Opera is just not very good).
The same goes for users who use Mozilla or Netscape 6+ (just click to the Mozilla icon and the same OSNews panel will appear to your Mozilla sidebar).
Please note that the same panel page (mozilla.php) serves both browsers pretty well. The page's got the name mozilla.php because we got first developed it for Mozilla/Netscape a few months ago (we added Opera support just today).