How do I add a Sharepoint Online calendar to Outlook 2016?

To share information between a SharePoint Calendar and Outlook, users will need to establish a connection between the SharePoint Calendar and Outlook. Once the connection is established, users can view and manage the SharePoint calendar directly from Outlook.

Connect SharePoint Calendar with Outlook:

1. Sign into your SharePoint site in a web browser, and navigate to the calendar that you'd like to connect to.
2. Click Calendar TAB, and then click Connect to Outlook. A window should pop up. This will vary based on the browser you are using; see the browser differences below. 
3. A window will pop-up in your Outlook Email Client asking you whether you want to connect this SharePoint calendar to Outlook (as shown below). Click Yes. 
4. Click Allow.
5. The SharePoint Calendar should now show up in your list of calendars in Outlook. You can add appointments to it there, just as you would to any other Outlook calendar.
6. See Below.