I think OZ uses Amadeus as Res system but has its own host system (I think IBM Res3 based). The 8 digit number is host reservation number.
This is not something unique only to OZ, e.g. IB has the same arrangement - Amadeus as Res system but Resiber as host/check-in system.
AFAIK OZ is fully Alteá, but the 8-digit number is part of the legacy system which has specifically been ported over to Amadeus.