Definition of p.o.

p.o.: Abbreviation meaning by mouth, orally (from the Latin "per os", by mouth). One of a number of hallowed abbreviations of Latin terms that have traditionally been used in prescriptions.

Some others:

a.c. = before meals (from "ante cibum", before meals)
b.i.d. = twice a day (from "bis in die", twice a day)
gtt. = drops (from "guttae", drops)
p.c. = after meals (from "post cibum", after meals)
p.r.n. = when necessary (from "pro re nata", for an occasion that has arisen, as circumstances require, as needed)
q.d. = once a day (from "quaque die", once a day)
q.i.d. = four times a day (from "quater in die", 4 times a day)
q._h.: If a medicine is to be taken every so-many hours (from "quaque", every and the "h" indicating the number of hours)

hope this explains

love liz