How do I sync with all the calendars in Outlook.com account?

In Android 4.4, it's now possible to show multiple calendars from Exchange

- https://digibites.zendesk.com/hc/en-us/articles/200299353-Does-DigiCal-display-multiple-Exchange-calendars-

* Since the recent Outlook.com app revamp, installing the Outlook app won't fix the issue anymore and you'll need to have Android 4.4+ and add your Outlook details as an Exchange account to see multiple calendars. Please follow the above link for more info.