Once you are logged into Office 365 (Outlook), navigate to Admin → Exchange → Resources. This allows you to create a new room resource. 

Note: A new room resource will be created with a max appointment time defaulted to 24 hours. 

You can view this after creating the resources by clicking on it in the list, selecting Edit, and then selecting booking options in the pop-up window menu. 

Warning: If you attempt to create a reservation through iOffice for that room that is longer than 24 hours (multi-day reservation), everything will appear fine on the iOffice end, but the exchange resource will decline the appointment. 

You can change this default setting to any value you wish (with 0 being an unlimited duration). This results in the multi-day reservations being accepted by the resource and the appointment will then appear on that resources calendar. 

One thing to note is that the declined appointment will still appear on the calendar of the user who tried to create it, if you click on it, you can see that the resource responded by declining. 

The appointment on the 16th and 17th in the above image has been declined by the room resource, but still appears on the calendar of the user. 

Note: It is not shown when you view the room's calendar (above image).