If you are logged into your user account and there is malware on it your system then it is compromised. It doesn't matter what web browser you are using. When you use your password to log into your bank or whatever it has to be decrypted to be entered on the webpage. During that time malware can steal it. It doesn't matter if its in a secure password manager or a web browsers password storage or anything. The only time these things can protect your data is when they are not being used.
Its like a lock on your door. Its only working when the door is closed. Doesn't matter how great the lock is if the door is wide open. From that perspective, yes, Opera is safe. The file that stores your data can't be used unless its being used from inside your Windows account. So it can't just be copied off your hard drive by a second operating system and read plain text. But if its accessed while your system is operating, like by malware or a hacker, you are wide open. As with anything else like that.