Open System Preferences
Click Internet Accounts.
On the account type page, click Exchange type. Continue to the next step.
Important: If you are not presented with the above screen, you may need to click on the "+" sign located at the bottom left of the "Accounts" window. This action will start the account creation process.
On the following page it will ask for your account credentials:
Click Continue
If you receive the following error - Click Continue.
If you are presented with the following screen fill in the required credentials.
The username field will automatically be populated with the email address you entered previously. This is not necessarily the address that should be used as the username. There are two types of accounts in Office 365 that determine the username that should be used.