To manually configure your Office 365 online account and set up the Delegation (Reviewer) rights using Outlook:
Create Office 365 Exchange Online account.
Go to File menu in Outlook.
From the Account Settings drop-down, click Delegate Access.

If there are no accounts, click Add to add the account.
Search the Exchange Online account and click Add.

Click OK. The Delegate permissions pop-up window is opened.
From the Calendar drop-down, select Reviewer (can read items).

Click OK.