SAP Fiori - Gateway Services for Approval Apps
This gives you details for OData in SAP Fiori Approve Purchase Order app. You see which OData is called from which UI. Understanding model provider class and data provider class helps for extending the standard app. There is example for adding additional fields by BADI. For details step for adding fields, please find "How to guide: SAP Fiori Extensibility - Adding custom fields in OData". Please visit SAP Fiori learning materials. http://scn.sap.com/docs/DOC-45780.