There are many differences between CDA and openEHR, the first difference is CDA is a definition of an XML format, while openEHR is a specification for a full EHR architecture. The second difference is CDA is only a document definition, while openEHR has other extra features like having EHR management and FOLDER management (CDA doesn’t have EHRs or FOLDERs).
At the document level, anything that could be done in CDA could be done in openEHR, but the other way around is a little tricky.
And lastly, the CDA model wasn’t designed to be archetypable, in openEHR you have clinical documents archetypable by design. CDA works with implementation guides, that are plain text documents, and openEHR relies on archetypes and templates, that are computer-processable definitions.
(disclaimer: I do workshops related to openEHR and to CDA )