Disclaimer: I work in product management at Red Hat, I would suggest reaching out to Mirantis directly to ask questions regarding this topic.

Now that we have that out of the way, my understanding is that Mirantis OpenStack continues to be a component of Mirantis Cloud Platform and will not be available separately for new customers, instead you consume it as part of MCP. The name change is the result of the other things being added (primarily StackLight and Kubernetes) as well as the significant changes to the way they are delivering/deploying/managing it (moving away from FUEL towards a CI/CD approach driven by a tool called Drivetrain).

From what I could gather during the introductory webinar sgordon's answer is correct.

It must be noted that whilst MCP retains a component called MOS, it's a completely different beast from the old MOS that used to be provisioned by Fuel/Puppet. I attended Mirantis' OpenStack BCN presentation on the architecture, which also featured a demo, useful for grasping how different - and indeed more advanced/complex - the MCP-provisioned MOS is.

My gut feel: MCP is a giant step in the direction of a much more supportable architecture (I happen to like Salt too, which is used instead of Puppet for provisioning) at the expense of a big uptake in complexity. Having a platform that is indeed upgradable is probably worth a few sacrifices on that front...

There are two important corollaries as far as I am concerned:

• Old Puppet/Fuel based MOS is being EOLed (though not before 2019 and possibly longer if you have a support contract)
• Unlike MOS, you cannot just download MCP and take it from a spin - with a view of perhaps enrolling into a support contract with Mirantis once your setup reaches critical mass. The consumption model involves upfront contracting and a minimal period of hand-holding, during which Mirantis mostly operate your platform for you; likely due to aforementioned complexity.

(Sorry for separate answer due to comment size limit)