These simple steps show how to add a Google calendar on Mac.
Install OneCalendar
Download OneCalendar from the store and install it on your device.
Select Accounts overview
Select the OneCalendar menu on the top left, and select the option "Accounts".
Select Add account
Click on the "+" sign to add a new account.
Select Account type
Click on the "Google" account type.
Enter credentials
A new browser window will be opened where you need to enter your Google credentials.