grammatical number - Plural form of Abbreviation Ending in O

My company deals in Purchase Orders.

For years they have referred to multiple purchase orders as POs. It seems, the proper spelling would be POes, because a P-O is referred to as a noun by everyone in the company, even though it is an abbreviated form of Purchase Order.

Which is correct? POs or POes ?

Answer

Because PO is an abbreviation, it just gets an -s: POs. It's not the same as potato or tomato, which would get -es.

See Oxford: "Just add -s (or -es, if the noun in question forms its plural with -es). For example: MPs ... An apostrophe should never be used to form the plural of ordinary nouns, names, abbreviations, or numerical dates."

So: POs because order takes a simple s. And not PO's with an apostrophe.

However, I believe the Chicago Manual of Style does advocate using an apostrophe (but I don't have a reference for that, if it's online at all).