For rentals the OP and PS statuses are most likely being used b/c the listing agent has received several applications and they "look" like one could be selected and approved as the tenant, but the processing and background checks for these applicants is not completed yet. But, since there is no approval, the property could still be shown to other prospects. P (pending) status is selected when an applicant has been approved but still has not moved in yet.
However, even with this said, the use and meaning of a particular status in the Rental category may vary slightly depending on the Realtor.
Just scroll on down and you'll find out. This is a questions that gets asked a lot! It can be a little confusing for consumers. I think HAR should explain this on the front page of their consumer sight if they haven't already. Have a fabulous weekend!