Warning when adding wrong stock type to PO

Is there a way to trigger a warning if you enter an incorrect stocking type on a PO.

We use OO type PO for outside processing and sometimes the purchaser manually adds these Purchase orders

Out of habit, he often will open a OP and add the *OP1 item to the PO.

Is there a way to prevent adding an X line type item to an OP PO type?