AFAIK, you must be able to consume the OData service SAP, in this case, make sure OData enabled external source type is configured to Integration Server with a valid connection.
Another option is to call the OData service (similar to Rest) by the client:HTTP.
I have created an “OData Service using an External Source Type” with valid JDBC connection only. There is no option to select SAP connection while creating Odata service using above option.
I have to integrate with SAP system only and I have the service provider URL.
AFAIK, OData feature provided by wM is not quite matured, you cannot consume the URL (like we do for WS-WSDL), if you are unable to use the SAP connection as an external source then only option left is to write wrapper services using client:http (_get,_post,_put,_delete)
Not sure if others have any idea, but lets wait and watch.