Oracle Stored Procedure If Not Exists Return

I've been a strictly MS SQL consumer, but have been tasked with writing an Oracle stored procedure that compares 2 tables for missing data. I need to write something that will return a 1 if an order is missing. So far I have the following, but it's not even close to working. I've scoured the web with little results. Any help would be greatly appreciated.

CREATE OR REPLACE PROCEDURE SP_PO_CHECK


AS

BEGIN

IF NOT EXISTS ( SELECT PO FROM ORDERS, PO_LIST, WHERE Order_PO = PO; ) THEN RETURN 0; ELSE RETURN 1

END;