Office 365 (Outlook 2016) - Meeting Invites Showing up on Calendar

By default, Outlook 2010 and Outlook 2013 will automatically process your meeting requests so these time-slots are reserved on your Calendar as Tentative. This is to prevent others from booking the same time-slot before you have processed an earlier sent meeting request.

Troubleshooting 
The automatic processing of meeting requests can be controlled via your Tracking options.

• File > Options > Mail > Tracking > disable: Automatically process meeting requests and responses to meeting requests and polls

While disabling this option will prevent time-slots from being booked automatically as Tentative, it also requires you to manually process any responses that you get to your invitations and polls.

In order to process the received responses, it is sufficient to view the response in the Reading Pane or to open the response via a double click.

Tracking options allow you to automatically process certain responses and requests.

Note 1: Even though a meeting request will no longer be automatically added as Tentative to your Calendar upon receiving, it will still be added as Tentative as soon as you select the meeting request in your message list.

Note 2: Disabling this option can also come in handy when you receive a lot of spam messages in the form of meeting requests.