The Collaborative International Dictionary of English v.0.48:

Phosphoryl \Phos"phor*yl\ (f[o^]s"f[o^]r*[i^]l), n. [Phosphorus
   + -yl.] (Chem.)
   The radical PO, regarded as the typical nucleus of certain
   compounds.
   [1913 Webster]

WordNet (r) 3.0 (2006):

P.O.
    n 1: a noncommissioned officer in the Navy or Coast Guard with a
         rank comparable to sergeant in the Army [syn: petty
         officer, PO, P.O.]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

PO
       Portable Object (MO)