Menu>Settings>Preferences>Advanced Tab> Scroll to the Security section.
Click on the Set Master Password button
Enter your Password > Confirm it and click on the OK button.
Now it still remains to define the frequency of use(in my case -Once per session)
Protecting the password of your online accounts
Under Opera (and most browsers) upon signin in on a website, your browser will automatically prompt you if you want to save these informations, for future use (auto-completion feature)
The nice trick on Opera is that you have the "Remember choice for this page option", that is once you have saved the information to login to a certain website a simple CTRL + Enter, will be sufficient to login to online resources (no need to type anything). Sure it's handy, but that also means that anyone can login to your account from your PC, during your absence.
Now it's time for the " Use master password to protect saved passwords" feature comes into play.
Simply check this option in the security section to enable it (Ask for password will automatically switch to the " Every time needed" status).
Here's an example of how it works, upon login an online resource using the CTRL + Enter key, here's what you shall get: