HL7 seems to be a standards organization whose goal is to standardize the domain of electronic healthcare support systems. The scope that HL7 aims to standardize is vast. It covers everything from Electronic Medical Record systems to Laboratory Imaging Systems.
There are orthogonal standards like ICD-10, SNOMED-DT etc, which standardize the terms used in clinical services. Its relation to HL7 is currently unknown.
Also unknown is the relationship between FHIR and HL7
Another unknown is how IHE fits into this picture, though from a preliminary glance, IHE seems to provide guidelines to technical implementations on the usage of these standards in their architecture. An aspect that is confusing with regards to IHE is its organizational structure. IHE is divided into domain specific committees. Whether this was has any implication on the architecture of the implementations is currently not known.
Once we have figured out the big picture of how one thing related to another, we would have to figure out which platform we are going to use to implement these services. OpenMRS seems to be the platform of choice and it seems to support expressing these concepts.