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

The oc get dns command can be used to list the DNS objects. By default, OpenShift should have a single DNS object named "cluster".

~]$ oc get dns
NAME      AGE
cluster   422d

The oc describe dns command can be used to display more details, including the Base Domain name.

~]$ oc describe dns cluster
Name:         cluster
Namespace:    
Labels:       <none>
Annotations:  <none>
API Version:  config.openshift.io/v1
Kind:         DNS
Metadata:
  Creation Timestamp:  2021-07-16T17:10:10Z
  Generation:          1
  Managed Fields:
    API Version:  config.openshift.io/v1
    Fields Type:  FieldsV1
    fieldsV1:
      f:spec:
        .:
        f:baseDomain:
      f:status:
    Manager:         cluster-bootstrap
    Operation:       Update
    Time:            2021-07-16T17:10:10Z
  Resource Version:  545
  UID:               8d2d9acf-2248-4b94-a241-0c824ec4edbd
Spec:
  Base Domain:  openshift.example.com
Status:
Events:  <none>

Or, the oc get dns command with the --output json​ or --output yaml option can be used.

~]$ oc get dns cluster --output yaml
apiVersion: config.openshift.io/v1
kind: DNS
metadata:
  creationTimestamp: "2021-07-16T17:10:10Z"
  generation: 1
  name: cluster
  resourceVersion: "545"
  uid: 8d2d9acf-2248-4b94-a241-0c824ec4edbd
spec:
  baseDomain: openshift.example.com
status: {}