When you sign in to Office 365 for the first time, you will need to authenticate yourself using another method in addition to entering your OST password. Please have your mobile phone ready for the following instructions!
Using your default web browser, open the Office 365 login page: https://portal.office.com
Important
If you see the Office portal below without signing in, then you are already signed in with another Office 365 account and you have to sign out first. After that, you can click forget this account and sign back in:
Sign in with your OST account:
Provide additional information to complete your registration. The easiest way is to have an SMS sent to your mobile phone:
Registration is complete.
Step 2: How do I change my OST password?