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.
If you go for an Office 365 plan, make sure to get Office 365 Business Essentials or Office 365 Business Premium. Note that Office 365 Business does not come with Exchange Online.
You cannot use an Outlook.com account to connect OnceHub to Office 365 Calendar, as Microsoft does not allow Outlook.com accounts to access the Office 365 Calendar API.
Verifying your account compatibility
The easiest way to verify you have an appropriate account is by logging into your Office 365 account and checking that Calendar is listed in your apps (Figure 1).