In other words, let OPES processor worry about its application input and output. OPES processor negotiates with the callout server what OCP application they should use and then handles necessary conversions, if any. Naturally, the processor should not negotiate OCP applications that it cannot convert input application to or output application from.
OCP application could be HTTP. Or it could be something like "transfer-encoded HTTP message body with content-type and transfer-encoding passed via meta-data". Etc...