During interoperability exchanges, organizations are jointly conducting computation and sharing tasks. However, organizations can have different security policies. To guarantee good interoperability exchanges, organizations need to share with other participants information about the services they provide. In addition, to be compliant with security requirements during interoperability, security policies have to be dynamic. One purpose of this paper is to provide this dynamic behavior by taking care about context of access parameters. The context-aware security requirements may be met by using a contextual access control model to define the security policy of each party involved in the interaction, and OrBAC (Organization based Access Control) is an adequate model for this purpose. Elaborating an ontology based security model provides a mean to ensure sharing of understandable knowledge, in particular knowledge needed to derive the authorized accesses and usages during the interoperability sessions. In this paper, we thus suggest a context ontology to be combined with an ontological representation of the OrBAC model and show how it can be used to ease the security rules definition and derivation during interoperability sessions.