HL7 OIDs and FHIR URIs

I am new to the Health Level 7 standards and messaging paradigm and was reading up on OIDS/URIs. While I know OID are unique Identifiers assigned under an Assigning Authority, I would like to know the following:

1. If there any difference in their usage for V2 and V3 messages.
2. Whatis actually sent over the wire in terms of an OID?
3. Why is there a switch from OIDs to URIs in the context of the new HL7 FHIR standard?

Any help is appreciated.

Thanks