Somewhere in the customer portal there should be a KS that provides that information, but if you have cluster-admin access to a cluster you can try:
$ oc debug node/name-of-the-node
...
# chroot /host
...
# cat /etc/system-release
...
RHEL_VERSION="8.2"
...
That was from OpenShift 4.5.4 in a DO280-v4.5 classroom.
CoreOS is built from RHEL rpms. You can even use the rpm command from an oc debug shell to see the exact package releases that are installed in a node.
Of course it is not just RHEL 8.2, it is 8.2 with all z-stream updates at the time the CoreOS image was built for that OpenShift release.
