Hi,
Unfortunately there is no add-on to just install, but here is a solution which is pretty close:
1. Find the folder storing the .ini files for your Opera browser. On Windows this folder is usually C:\Program Files\Opera\defaults. Depending on your OS just locate your Opera main folder and navigate to defaults.
2. Save the urifilter.ini file from this address:
http://www.technobeta.com/download/urlfilter.ini
in the folder
This is all! Opera will now block the main ads present on the Internet.
! the defaults folder gets overwritten at each update, so make sure you repeat the process when you install a new Opera version
!! The urifilter.ini file at
http://www.technobeta.com/download/urlfilter.ini gets updated quite often so you might want to repeat the process from time to time to make sure you keep all those annoying ads away
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This is the best solution which Opera can offer, though it's not by far as efficient as the most common Firefox addons..
Hope this helps,
wbwTeam