To be able to connect OnceHub to your Office 365 Calendar, you must have a calendar in Outlook on the web for business, which is provided with Exchange Online. Exchange Online is available as a standalone service, or you can get it as part of an Office 365 plan.

Verifying your account compatibility

The easiest way to verify that you have an appropriate account is by logging into your Office 365 account and clicking All apps (Figure 1).
Figure 1: All apps

Then, make sure that Calendar is listed in your apps (Figure 2).

Figure 2: Calendar in the Office 365 account app page