BioPortal Biomedical Ontology - http://bioportal.bioontology.org/

Ontology for General Medical Science - https://code.google.com/p/ogms/

Open Clinical - http://www.openclinical.org/ontologies.html

OpenGalen http://www.openclinical.org/

Have a look at the HL7 reference information model. Arguably more of a data model than an ontology and certainly not RDF or OWL , but in real use for describing what has been examined, what has been diagnosed, what medicine has been prescribed etc.

In a different direction, there is MeSH (Medical Subject Headings), a medical thesaurus that is geared towards medicine, but since medicine borders on lots of other things, has entries for everything and the kitchen sink. May be available or at least expressible in SKOS.

Hello,

I finally opted for HL7 Standard

Thank you for all your help !

have you tried swoogle? there you can find ontologies on different domains.

HL7 reference model is going to be your best bet from a short and long term POV given the interchangeability that products have with it.