Hi Ricky,
Use IBASE instead of ONEORDER and root object name IBHeader instead of BTOrder and then guid would be the IBase Guid. After you get root enetity you can check the corresponding related entity by using the tcode GENIL_MODEL_BROWSER.
Try playing around these 2 tcodesGENIL_MODEL_BROWSER and GENIL_BOL_BROWSER you will get to know more about these relations and stuff.
Best Regards,
Dharmakasi.