I’m not sure why that nested structure exists. Is it standard for HL7 OML^O21? If it is, Intersystems should ideally provide a clearer error message to help us troubleshoot this more effectively.

It is part of the HL7 v2.4+ specification for the OML_O21 message structure. The nested segment group is for prior patient visit, order and observation data. 

When I run the message through Interoperability | Interoperate | HL7 v2.x | HL7 v2.x Message Viewer with the following options:

I get the following errors:

Segments required by the HL7 2.5 OML_O21 specification are not present and the message is failing to parse after the first ORC. You can create a custom Doc Category and DocType Structure to eliminate the required segment(s) and associate it with the OML_O21 message type (alternately, you can just assign the ORM_O01 Structure to the OML_O21 message type).

Then use that custom Doc Category as the Message Schema Category in the inbound service and Doc Category/Doc Type in your DTL.

Without seeing the source message, it will be difficult to provide a definitive answer. My guess would be that there are segments in the message that would normally appear in an OBRgrp (or sub-group) but no OBR segment is present.

Also ... your screenshot indicates that the target message type/trigger event is being set to OML^O21, not ORM^O01. This differs from the description.