I have seen that before, update your NIC firmware and that should fix it.  It is a good practice to update all a server's drivers and firmware before installing OMSA, but usually the problem is not that severe.
I installed Openmange 6.5.0 on a Dell T300 this week and it immediately drops the network connection upon installing. The network adaptor indicates that it cannot detect the connection type and so it will not allow connections. When I uninstall the manager, the connection is restored and the adapter recognizes the connection as a domain connection. I am running Server 2008 R2 standard and this a member server of the local domain.
Openmanage 6.5.0 stops network connection