Now I'm not sure, but I beleave the account used as login to oracle is the one which runs the service not the one which has called the service.
You should look into impersonation
So in you WCF method do something like the below (taken from the site I linked):
WindowsIdentity callerWindowsIdentity = ServiceSecurityContext.Current.WindowsIdentity;
if (callerWindowsIdentity == null)
throw new InvalidOperationException("The caller cannot be mapped to a WindowsIdentity");