In order for applications to be able to detect that calendars belong to resources and/or rooms, there is a specific configuration requirement for Office 365 and Exchange.

To create a Room or Resource you can use the Admin Web Interface for Office 365: https://portal.office.com/adminportal/home#/ResourceMailbox

Rooms and resources must also be added to a Room List, but unfortunately Office 365 does not give the option to do this via the Admin Web Interface.

It is, however, possible via Powershell.

You will first need to connect to your Office 365 instance as detailed here: https://technet.microsoft.com/en-us/library/jj984289(v=exchg.160).aspx

Once you have done that you can issue the Powershell commands required to create a Room List and add your Resource to it.

Creating a Room List

To create a collection of rooms called “Meeting Rooms” issue the following Powershell command:

New-DistributionGroup -Name "Meeting Rooms" -RoomList

Adding an existing Resource to a Room List

For an existing Room named “Board room” and an existing Room List named “Meeting Rooms” issue the following Powershell command:

Add-DistributionGroupMember –Identity "Meeting Rooms" -Member "Board room"

While a room list can contain as many rooms as you want, there is a limit that only a maximum of 100 items will be returned.
To avoid this you can create multiple room lists, each with less than 100 rooms.