Using RHEL 5.11
Built openscap-1.1.1

xccdf.xml source from 
Downloaded "Red Hat 5 Manual STIG - Version 1, Release 8"

That contains the U_RedHat_5_V1R8_Manual-xccdf.xml file.

I just tried running it, and all results show "notapplicable"

Looking through the docs, it does seem to point out, and google seems to also point out that I need to be running on the right platform.  It seems I am.

This is truly a Redhat 5 workstation(or server), not CentOS.

My command looks like this
oscap xccdf eval --profile MAC-1_Public --results result.xml U_RedHat_5_V1R8_Manual-xccdf.xml

The zip file from iase website didn't contain a CPE file, so I assumed one was not needed? Though I have a feeling this is my problem.

NOTE, that when I running I do see this message to stderr
WARNING: Skipping ./oval.xml file which is referenced from XCCDF content

Really hoping to get this working, this seems a great tool.