'P.O.' originates from English, specifically the phrase 'post office', where 'post' comes (via Old French 'poste' from Late Latin 'posta') meaning 'relay station' and 'office' comes from Latin 'officium' meaning 'service/duty'. The abbreviation is also used for 'purchase order', from 'purchase' (Old French) and 'order' (Latin 'ordo').

'post office' developed in Early Modern English to denote facilities handling mail; the abbreviated form 'P.O.' appears in 19th-century postal documentation and signage. The commercial term 'purchase order' became standardized in modern business practice, giving rise to 'P.O.' as shorthand in business contexts.

Initially, 'post' referred to relay stations for couriers and 'post office' to places for courier posts; over time these broadened to mean the modern mail/postal facility. 'P.O.' initially referred mainly to 'post office' but later expanded to other abbreviations like 'purchase order' or 'probation officer' depending on context.