The Output Post Processor (OPP) is an enhancement to Concurrent Processing and is designed to support XML Publisher as post-processing action for concurrent requests. If a request is submitted with an XML Publisher template specified as a layout for the concurrent request output, then after the concurrent manager finishes running the concurrent program, it will contact the OPP to apply the XML Publisher template and create the final output.
The Output Post Processor makes use of the Oracle Streams Advanced Queuing (AQ) database feature. Every OPP service instance monitors the FND_CP_GSM_OPP_AQ queue for new messages and this queue has been created with no value specified for primary_instance (link). This implies that the queue monitor scheduling and propagation is done in any available instance. In other words, ANY OPP service instance may pick up an incomming message independent of the node on which the concurrent request ran.