If you need to add a new calendar in OWA (for instance a room or an equipment calendar), just following the steps described bellow:
At the Office 365 Portal, clic the Calendar tile from the Office 365 Home or the App Launcher:
Once you are in the Calendar page, clic “Add calendar” link and choose one of the options available to add a new calendar:
In my case, I have selected the option of adding a calendar from the directory so I will have to specify the e-mail of the user or resource whose calendar I want to add:
To finish, simply clic “Open” button and you are done.