Thanks for replying my question.
Actually, all our computers have fixed IP, although it looks like some computers might have a second network card that is misconfigured as the IP range is given by the Microsoft DHCP service.
I don't know if it's possible to setup the desired network card as default for the OCS server or we will have to disable those misconfigured cards on the computers.
Also, some computers with just one network card with a fixed IP are having the same issue and their IP appear as given by the Microsoft DHCP service.
Any help would be gladly appreciated.