Open the Opera browser, and click the red "Menu" icon in the top-left corner of the program. Click "Settings" followed by "Preferences." Click the "Advanced" tab; then click "Security" in the left pane.
Click "Set Master Password" if you haven't already configured one. Type your desired password twice on the prompt that appears; then click "OK" to set the new password. Click the box beside "Use Master Password to Protect Saved Passwords" if you want to enable password protection for your saved website passwords too. Type your master password, and click "OK" to confirm.
Select a frequency to ask for the password from the "Ask for Password" drop-down menu. Select "Every Time Needed" to always ask for a password or "Once Per Session" to ask once during each browsing session. You can also select one of the additional frequency options, ranging from every 10 minutes to every hour, when you wish to receive a password prompt. Click "OK" to save your settings.