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

Let's say you have a deployment named "my-deployment".

~]# oc get deployments
NAME            READY   UP-TO-DATE   AVAILABLE   AGE
my-deployment   1/1     1            1           8d

The oc set volume command can be used to mount an empty directory in the container.

~]$ oc set volume deployment my-deployment --add --type emptyDir --mount-path /var/empty
info: Generated volume name: volume-879rj
deployment.apps/my-deployment volume updated

A new pod should immediately be created and the oc describe pod command can be used to see that the pod is now has the "foo" and "bar" keys.

~]$ oc describe pod my-pod-65rbl
Volumes:
  volume-879rj:
    Type:       EmptyDir (a temporary directory that shares a pod's lifetime)
    Medium:     
    SizeLimit:  <unset>

The oc exec and df command can be used to see that the empty directory has been mounted in the container.

~]$ oc exec pod/my-pod -- df
Filesystem                             1K-blocks     Used Available Use% Mounted on
overlay                                125277164 47124096  78153068  38% /
tmpfs                                      65536        0     65536   0% /dev
tmpfs                                   12333448        0  12333448   0% /sys/fs/cgroup
shm                                        65536        0     65536   0% /dev/shm
tmpfs                                   12333448    70460  12262988   1% /etc/hostname
/dev/mapper/coreos-luks-root-nocrypt   125277164 47124096  78153068  38% /var/empty
tmpfs                                   12333448       32  12333416   1% /run/secrets/kubernetes.io/serviceaccount
tmpfs                                   12333448        0  12333448   0% /proc/acpi
tmpfs                                   12333448        0  12333448   0% /proc/scsi
tmpfs                                   12333448        0  12333448   0% /sys/firmware