RFC 2618 defines the RADIUS-ACC-CLIENT-MIB, which describes the statistics of the accounting server that communicates with the RADIUS client.
The OID of the root object is:
iso(1).org(3).dod(6).internet(1).mgmt(2).mib-2(1).radiusMIB(67).radiusAccounting(2).radiusAccClientMIB(2).radiusAccClientMIBObjects(1).radiusAccClient(1)