What it means
A Latin phrase meaning through the mouth, the source of the abbreviation PO. ISMP lists "per os" as error-prone because the "os" has been read as OS, the abbreviation for the left eye, so an oral liquid could end up in the eye. Write "PO", "by mouth" or "orally" instead.
For the nurse
If an order says "per os", clarify it with the prescriber rather than guess, and never use it in your own notes.