1. Go to outlook.office365.com
  2. Enter your email address followed by your password. Note: if you're using Google Chrome as your browser and are connected to the company network, you won't need to enter your password.
  3. Once logged in, click on the avatar at the top-right of the screen
  4. Select 'Open another mailbox'
  5. In the subsequent pop-up, enter the name (or email address) of the mailbox you wish to open
  6. You now have access to another mailbox in OWA

    A video showing the process is below.