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

Let's say the oc get cronjobs (or oc get cj) command lists the following cron job.

NAMESPACE   NAME    SCHEDULE       SUSPEND   ACTIVE   LAST SCHEDULE   AGE
my-project  my-job  01 06 * * 1-6  False     0        21h             77d

The oc create job command can be used to manually run a cron job. 

oc create job --from cronjob/my-job my-job-manual-run