See link ..
http://artykul8.com/2011/06/moving-browser-cache-folder/
Also this is done thru Opera:Config
Opera:
Review the location of the current cache folder by typing in the URL: opera:about or from Menu Help About Opera.
To modify the settings type the URL:
opera:config
Navigate to User Prefs tab and modify its cache folder:
Cache Directory4=d:\Temp\Opera\
(alternative quick jump link: opera:config#UserPrefs|CacheDirectory4)
Additionally you can modify temporary download folder for Opera as well, through another key:
Temporary Download Directory = d:\Temp\Opera
Operas Settings File Explained