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Definition of op. 1  1 chiefly US : a set of planned actions for a particular purpose âusually plural military ops [=operations] 2  2 : op art. 3  3 British, informal : a medical operation â see also photo op.
Terminology [edit]. Per os (/ËpÉrËoÊs/ ; P.O.) is an adverbial phrase meaning literally from Latin by opening or by way of the opening.. The expression is used in medicine to describe a treatment that is taken orally. The abbreviated P.O. is often used on medical prescriptions.