The master password under the Opera browser protects the user sensitive informations such as user saved passwords from being copied/accessed locally. Now what happens if you forget your Opera master password? In particular, nothing bad but Opera will not fill out the username and password of sites that have been stored.
Fortunately, the master password can be reseted and the problem solved.
Go to Tools >> Preferences >> Advanced >> Security, if “Use master password to protect saved passwords” is not enabled, simply delete the file opcert6.dat from the profile directory after you have closed Opera.
If “Use master password to protect saved passwords” is enabled, just delete the files opcert6.dat and wand.dat from the profile directory after you have closed the browser.
The Opera profile directory is located under “Documents & Settings” unless not pointed different by the user.