I don't see it mentioned anywhere in the 2.0 spec, is nonce not used by OAuth 2 and if not, now does it prevent replay attacks?
The 1.0 spec states:
3.3. Nonce and Timestamp
The timestamp value MUST be a positive integer. Unless otherwise specified by the server's documentation, the timestamp is expressed in the number of seconds since January 1, 1970 00:00:00 GMT.
A nonce is a random string, uniquely generated by the client to allow the server to verify that a request has never been made before and helps prevent replay attacks when requests are made over a non-secure channel. The nonce value MUST be unique across all requests with the same timestamp, client credentials, and token combinations.
To avoid the need to retain an infinite number of nonce values for future checks, servers MAY choose to restrict the time period after which a request with an old timestamp is rejected. Note that this restriction implies a level of synchronization between the client's and server's clocks. Servers applying such a restriction MAY provide a way for the client to sync with the server's clock; alternatively, both systems could synchronize with a trusted time service. Details of clock synchronization strategies are beyond the scope of this specification.