After using Opera for a while, open up a console and type in:
find /var -cmin -5
find ~/.kde -cmin -5
find ~/.gnome2 -cmin -5
This will tell you which files were created or changed in the last 5 minutes and will give you a clue which files you want to clear or delete.
Some may have entries that you may want to clear, rather than deleting the entire file.
Certain file name patterns, or directories could be cleaned out with a script, for others you may need to use 'sed' to replace "<name>=<value>" entries with <name>="".