@q53649 All password managers that are built-in to the browser are vulnerable to exploits if you have malware on your computer. But if your computer is "clean" the Opera password manager is as secure as any other browser.
The passwords are stored locally.
But you can also choose to have them uploaded (with an extra password to encrypt them) to the Opera servers for the convenience of them then being available on other devices or computers with Opera.
Syncing password to the Opera server is nice, but it is essential to have a strong password for the sync, and again, be careful about potential malware or bad extensions.
Personally, I don't store my banking and other financial passwords in the Opera password manager.