The Machine Config Operator (MCO) uses a boot image to start a Red Hat Enterprise Linux CoreOS (RHCOS) on each OpenShift node.

If you are not familiar with the oc command, refer to OpenShift - Getting Started with the oc command.

The oc get ClusterOperators command can be used to list the Cluster Operators, which should include the machine-config Operator.

~]$ oc get ClusterOperators
NAME                                       VERSION   AVAILABLE   PROGRESSING   DEGRADED   SINCE   MESSAGE
authentication                             4.20.14   True        False         False      209d
baremetal                                  4.20.14   True        False         False      699d
cloud-controller-manager                   4.20.14   True        False         False      699d
cloud-credential                           4.20.14   True        False         False      699d
cluster-autoscaler                         4.20.14   True        False         False      699d
config-operator                            4.20.14   True        False         False      699d
console                                    4.20.14   True        False         False      304d
control-plane-machine-set                  4.20.14   True        False         False      317d
csi-snapshot-controller                    4.20.14   True        False         False      304d
dns                                        4.20.14   True        False         False      304d
etcd                                       4.20.14   True        False         False      699d
image-registry                             4.20.14   True        False         False      304d
ingress                                    4.20.14   True        False         False      2d1h
insights                                   4.20.14   True        False         False      699d
kube-apiserver                             4.20.14   True        False         False      699d
kube-controller-manager                    4.20.14   True        False         False      699d
kube-scheduler                             4.20.14   True        False         False      699d
kube-storage-version-migrator              4.20.14   True        False         False      47h
machine-api                                4.20.14   True        False         False      699d
machine-approver                           4.20.14   True        False         False      699d
machine-config                             4.20.14   True        False         False      637d
marketplace                                4.20.14   True        False         False      699d
monitoring                                 4.20.14   True        False         False      317d
network                                    4.20.14   True        False         False      699d
node-tuning                                4.20.14   True        False         False      14h
olm                                        4.20.14   True        False         False      2d
openshift-apiserver                        4.20.14   True        False         False      289d
openshift-controller-manager               4.20.14   True        False         False      304d
openshift-samples                          4.20.14   True        False         False      408d
operator-lifecycle-manager                 4.20.14   True        False         False      699d
operator-lifecycle-manager-catalog         4.20.14   True        False         False      699d
operator-lifecycle-manager-packageserver   4.20.14   True        False         False      2d16h
service-ca                                 4.20.14   True        False         False      699d
storage                                    4.20.14   True        False         False      304d

The oc get nodes command can be used to list the nodes in your OpenShfit cluster.

~]$ oc get nodes
NAME                  STATUS   ROLES            AGE    VERSION
my-node-edge-lm6wz     Ready    infra,worker     519d   v1.33.6
my-node-edge-pmlls     Ready    infra,worker     519d   v1.33.6
my-node-infra-c4v5h    Ready    infra,worker     519d   v1.33.6
my-node-infra-mc8rc    Ready    infra,worker     519d   v1.33.6
my-node-infra-p9cjv    Ready    infra,worker     519d   v1.33.6
my-node-master-0       Ready    master           522d   v1.33.6
my-node-master-1       Ready    master           522d   v1.33.6
my-node-master-2       Ready    master           522d   v1.33.6
my-node-worker-lk5vm   Ready    compute,worker   61d    v1.33.6
my-node-worker-pj4r4   Ready    compute,worker   61d    v1.33.6

The YAML or JSON of each node should list the Operating System being used by the node.

~]$ oc get node my-node-infra-c4v5h --output yaml | grep -i osImage
    osImage: Red Hat Enterprise Linux CoreOS 9.6.20260204-0 (Plow)

When upgrading OpenShift, the boot image is NOT updated. 

If your OpenShift cluster is running on Google Cloud Platform (GCP) or Amazon Web Services (AWS), you can configure the boot image that is used in MachineSets. The oc get MachineSets command can be used to list your Machine Sets.

~]$ oc get machinesets --namespace openshift-machine-api
NAME                DESIRED   CURRENT   READY   AVAILABLE   AGE
my-cluster-edge     2         2         2       2           140d
my-cluster-infra    3         3         3       3           140d
my-cluster-worker   2         2         2       2           143d

In this example, the Machine Set is configured to use the Amazon Web Services (AWS) Amazon Machine Image (AMI) ami-0xyz987abc852d123.

~]$ oc get MachineSet my-cluster-infra --namespace openshift-machine-api --output yaml
spec:
  template:
    spec:
      providerSpec:
        value:
          ami:
            id: ami-0xyz987abc852d123