To specify a storageclass as the default you must add the proper annotation to the storageclass definition:
metadata:
annotations:
storageclass.kubernetes.io/is-default-class: 'true'
This can be accomplished in multiple ways.
Using the WebUI
Select Storage -> StorageClasses -> Actions -> Edit annotations

- Select Add More
- For Key enter: storageclass.kubernetes.io/is-default-class
- For Value enter: true
- Save
Using the oc command to patch the resource
# Get a list of the storage classes
oc get sc
# Patch the storage class
oc patch storageclass <storageClass Name> -p '{"metadata": {"annotations": {"storageclass.kubernetes.io/is-default-class": "true"}}}'
# Verify that it has been marked as default
# the NAME will be followed by (default)
oc get sc