O/S means Out of Stock. It is only applicable for items that can't be back-ordered. In the Item Master, if the "17. Backorderable?" flag is set to "N," then it won't be possible to back-order that item. Generally speaking, you won't be able to back-order items that are being phased out or obsoleted. Sometimes, a special one-time-deal item can't be back-ordered either. You do not want these types of items to be back-ordered because once the current inventory is sold, there will be no more to sell even if your customers are willing to be on back-order. By default, the Backorderable flag in the Item Master file is set to "Y." If the "Backorderable" flag is set to "N" in the Item Master file, then the line item will close automatically after an invoice is posted even if the line item is not shipped or partially shipped.
Typically, when you run out of an item ordered by a customer, you will enter 0 in the QTY-TO-SHIP field. The system will then indicate that this item is either B/O (Back-Ordered) or O/S (Out of Stock). The key is the Backorderable flag in the Item File. If the Item File indicates that this item cannot be back-ordered, then the system should display O/S instead of B/O. Also, at the order line item level, you can mark an item that can be back-ordered to one that is non-back-orderable. This is an override that you can perform on a line-item by line-item basis.
As shown in the example above, if the user presses "F3=Not To Backorder" in the "Unit Price" field, then the line item becomes an item that cannot be back-ordered. A user can do this, even though this item can actually be back-ordered. This type of situation might occur if a customer chooses not to be on back-order for an item, even though the item itself can be back-ordered.