....
>

>m.xmlDoc.SetProperty("SelectionNamespaces", 'xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"')
>
>m.oNode = m.xmlDoc.SelectNodes("//cac:OrderReference")
>

>
>b) using an XPath function to retrieve element names, regardless of their namespace:
>

>m.oNode = m.xmlDoc.SelectNodes("//*[local-name() = 'OrderReference']")
>

>
>My advice? Always use the namespace selection.

Had similar SWAG on the reason for error, verified by inserting

? oNode.length

then I refreshed this thread, saving me from further effort as you had already forged ahead

But my advice would be slightly different:
Get away from XML and its intricacies, move to JSON for KISS reasons.
;-)