Companies typically use the term "open purchase order" to refer to any PO that has been created and issued to the vendor, and an "outstanding purchase order" would be one that is still open past its due date. So an outstanding PO would also be an open PO. "Outstanding" implies that there is some action required. There may be a specific technical definition for these terms within your accounting system, so you should ask the person who manages that system. It's a legitimate question, even for a native speaker.