University of Oslo is now making two-factor authentication (2FA/MFA) mandatory for Outlook for Mac.
You will get an error message in Outlook, which says that your username, password security settings are incorrect. Press Yes.
A dialog box appears, where you are asked to enter your username and password. To activate the new two-factor authentication security setting, it is important that the username is entered in the form username@uio.no.
Choose which account you want to log in with.
Then enter your password, and click Sign in.
A window for Number matching will appear.
Enter the same two-digit number in Microsoft Authenticator to complete the number matching, and at the same time ensure that the authentication request comes from Microsoft Office. Finish with pressing Yes.
Quit Microsoft Outlook for Mac (File → Quit or cmd + q), then restart the program.