Both are correct. OK (two capital letters, no periods) is the form recommended by the AP Stylebook and listed as the primary entry in Merriam-Webster. Okay (spelled out) is equally standard and is the form book publishers tend to favor, particularly for verb use. If you follow a style guide, use whatever it specifies. If you don’t, either works — just pick one and stick with it.
What follows is the full picture: the genuine difference between the two, what each style guide actually says (including the part most articles get wrong), and the surprisingly good story of where this word came from.
The Origin of OK — And Why It Matters
Most people assume okay came first and OK is the abbreviation. It’s the other way around.
On March 23, 1839, the abbreviation “o.k.” appeared in the Boston Morning Post in a satirical jab by editor Charles Gordon Greene at a rival Providence newspaper. It stood for “oll korrect” — a deliberate, jokey misspelling of “all correct.” This was part of a linguistic fad sweeping educated young Americans, who abbreviated deliberately misspelled phrases for slang, much the way LOL and OMG work today. HISTORYHISTORY
The abbreviation might have died with the fad if not for a presidential election. In 1840, supporters of President Martin Van Buren — nicknamed “Old Kinderhook” after his hometown in Kinderhook, New York — adopted “Vote for OK” as a campaign slogan and organized OK Clubs across the country. Van Buren lost the election, but the slogan sent OK into the national vocabulary. While every other abbreviation from that Boston craze (KG for “know go,” OW for “oll wright”) vanished, OK survived because it got a second life as a political rallying cry right at the moment it might otherwise have faded. Smithsonian Magazine
The word’s origins remained disputed for more than a century until Columbia University linguist Allen Walker Read uncovered the true etymology in the 1960s through research in early newspaper archives. His work is now the accepted scholarly account, cited by Merriam-Webster, Smithsonian, and the Oxford English Dictionary. HISTORY
Okay, the spelled-out version, appeared in print some decades after OK and developed as a phonetic rendering of what the letters sound like when you say them aloud. According to the Oxford English Dictionary, the okay spelling emerged after a few decades, eventually becoming standard enough that both forms now coexist in every major dictionary. Grammarly
What the Style Guides Actually Say
The AP Stylebook: OK Only
The AP Stylebook is unambiguous: use OK, all caps, no periods. This applies to all forms of the word. The AP Stylebook requires OK even in verb forms like OK’ing — apostrophe and all. Because AP sets house style for American journalism, PR, and most news writing, OK is the dominant form in those contexts. Grammarly
The Chicago Manual of Style: Both, With a Telling Preference
Chicago is officially neutral — it accepts both spellings and does not mandate one. But the official Q&A reveals something more useful than a neutral stance. CMOS editors noted that the one time the term appears in their own explanatory text, they chose “okay,” because it looks more like a real word. This is why book publishers and fiction editors typically default to okay: CMOS doesn’t require it, but the people who wrote CMOS reach for it when writing for themselves. The Chicago Manual of Style
Merriam-Webster: OK as Primary, Okay as Equal Variant
Merriam-Webster lists OK as the main entry, with okay designated an equal variant. The dictionary follows the general principle that the first-listed spelling is recommended, but the CMOS Q&A notes that when two spellings are “equal variants,” the distinction is one of preference rather than correctness. The Gregg Reference Manual takes a practical editorial position: “In sentences, the forms okay, okayed, and okaying look better than OK, OK’d, and OK’ing, but the latter forms may be used.” Writers Relief
Usage in Practice
Data from Google Books shows that in overall English, OK overtook okay in 1990 and continues to be more common — but in fiction specifically, okay is the dominant spelling, reflecting the preference among book publishers and literary editors who follow CMOS conventions. Quick and Dirty Tips
OK and Okay as Parts of Speech
One practical reason to know both spellings: OK and okay operate across all major parts of speech, and the choice of form can affect how natural a sentence reads.
As an adjective: The most common use — describing something as acceptable or adequate.
The first draft was okay, but the second one was much stronger.
Is it OK to leave a few minutes early?
As an adverb: Modifying a verb to mean adequately or acceptably.
He did okay on the exam given how little time he had to prepare.
Everything went OK until the third quarter.
As a noun: The approval or green light for something.
She gave the OK before anyone else had even seen the proposal.
They’re still waiting on the okay from legal.
As an interjection: Signaling agreement, transition, or acknowledgment.
OK, let’s get started.
Okay, I hear you — I’ll look into it.
As a verb: Here is where spelling choice becomes genuinely practical. When you need past tense or a gerund, okay produces cleaner forms.
She okayed the purchase order before noon. (preferred by most editors)
She OK’d the purchase order before noon. (AP style; requires the apostrophe)
The past tense of OK, per AP Stylebook recommendation, is OK’d — with an apostrophe. Okay simply becomes okayed. For most non-AP writing, okayed and okaying are the cleaner choice and raise no editorial objections. The Writer
Which Spelling Should You Use?
Use OK if:
- You follow AP style (journalism, news writing, PR)
- You want the form most common in overall American English usage
- You need a slightly more concise or direct tone
Use okay if:
- You write fiction, memoir, or book-length work under CMOS
- You need it as a verb and want to avoid OK’d and OK’ing
- Your house style or editor specifies it
- You simply prefer the form that reads as a complete word
Avoid lowercase ok in any formal or edited writing. It is widely understood in casual messages and texts, but most style guides and editors treat it as nonstandard in anything professionally produced.
Avoid O.K. with periods. The periods were common in early usage but are now considered unnecessary and dated. No major current American style guide recommends them.
One firm rule for every context: choose one form and use it throughout your document. Mixing OK in one paragraph and okay in the next looks inconsistent and will get flagged in a professional edit.
When Neither Form Is Right
Both OK and okay carry an informal register. In very formal writing — legal contracts, certain academic genres, official government documents — neither may be appropriate. In those cases, consider a more precise synonym: acceptable, approved, satisfactory, adequate, permitted, or authorized. The right choice depends on what you’re actually trying to say: approval (the contract was approved), adequacy (the performance was satisfactory), or permission (you are authorized to proceed).
FAQs
Which came first, OK or okay?
OK came first. Most modern reference works trace it to an 1839 Boston newspaper as an initialism of “oll korrect,” a misspelling of “all correct.” Okay developed later as a phonetic spelling of the letters O and K. Despite what many writers assume, okay did not produce the abbreviation — it was the other way around. Wikipedia
Does the AP Stylebook prefer OK or okay?
The AP Stylebook specifies OK — all caps, no periods. That preference extends to verb forms: the past tense is OK’d (with an apostrophe), and the gerund is OK’ing. If you write for a newspaper, wire service, or publication following AP style, use these forms consistently.
What does the Chicago Manual of Style recommend?
Chicago officially accepts both and does not mandate one over the other. However, the CMOS editors noted in their Q&A that when writing in their own explanatory text, they chose okay because it looks more like a real word. Most book publishers and fiction editors using CMOS default to okay in practice.
Is okay better than OK as a verb?
For most non-AP writing, yes. Okayed and okaying read more naturally than OK’d and OK’ing, and most editors outside journalism prefer the spelled-out forms. If you follow AP style, use OK’d and OK’ing as specified.
Is lowercase ok acceptable in formal writing?
No. Lowercase ok is appropriate in text messages, casual chats, and informal digital contexts. In any professionally edited or formal writing, use OK or okay.
What about O.K. with periods?
It is not wrong in a strict grammatical sense, but it is outdated. The periods were common in the 19th and early 20th centuries and appear in historical texts. No major current American style guide recommends them, and most modern editors will remove them.
Are there contexts where both spellings are too informal?
Yes. In very formal writing — legal documents, certain academic or official contexts — consider a synonym instead: acceptable, approved, satisfactory, or authorized. The right word depends on whether you mean approval, adequacy, or permission.
The Bottom Line
OK is the older form, the AP-specified form, and the more common form in overall English usage. Okay is the equal variant that CMOS editors reach for in their own text and that book publishers tend to favor, especially as a verb. Both are fully correct in standard American English. Lowercase ok and periods in O.K. are both outdated in formal contexts.
Follow your style guide if you have one. If you don’t, choose based on your context: OK for journalistic or punchy writing, okay for literary or book-length work, and a formal synonym when neither feels right for the register. Whichever you choose, apply it consistently from first sentence to last.
Conclusion
Both OK and okay are standard in American English. AP Style and Merriam-Webster list OK as the primary form. Chicago accepts both without a stated preference.
For most professional writing, OK is the safe default. When you need it as a verb, okayed and okaying make okay the more practical choice. Either way, skip the periods in O.K. and keep lowercase ok out of anything formal.
Pick one form, apply it consistently, and move on.
