This document specifies the core of the Open Pluggable Edge Services (OPES) Callout Protocol (OCP). OCP marshals application messages from other communication protocols: An OPES intermediary sends original application messages to a callout server; the callout server sends adapted application messages back to the processor. OCP is designed with typical adaptation tasks in mind (e.g.@ virus and spam management@ language and format translation@ message anonymization@ or advertisement manipulation). As defined in this document@ the OCP Core consists of application-agnostic mechanisms essential for efficient support of typical adaptations.