The OCM100 has recently been updated to the Ocata release by Mirantis, our course is on the Liberty release. There are no prerequisites beyond a knowledge of linux, networking and OpenStack.
So what is the diffrenet between two release?
OCM is specific to the Mirantis OpenStack version, if I remember correctly, and the COA is from the OpenStack Foundation and is not vendor-dependent.
Ocata is a more recent release of OpenStack then Liberty. Each OpenStack release is voted upon by the community and is in alphabetical order and related to the city in which the summit takes place. For reference Rocky is the most recent release and we are currently working on Stein.
As far as differences between the releases, there are additional features added by the different projects that you can find within their release notes and occassionally features are deprecated or retired which again can be found in the release notes.