Because of the Microsoft Software License Terms for Office Web Apps and Office Online Server, the server hosting Office Online Server must be dedicated to the customer, which excludes any IaaS (Infrastructure as a Service) providers that would not meet this requirement.
The software that is currently supported on IaaS can be found at Microsoft server software support for Microsoft Azure virtual machines.
For more information, see Volume Licensing for Microsoft Products and Online Services.
On the reference page, under Product Terms, choose the Language and Download. Open the Word document that contains the Product terms. Office Online Server is listed under Office Desktop Applications. The relevant section is under Software Assurance: "License Mobility: N/A."
Then, review what License Mobility means: License Mobility through Microsoft Software Assurance
This concerns the applications that are allowed to run on IaaS.