OK vs O.K. vs Okay vs ok – Which do I use?

In short, they’re all acceptable.

I was taught to spell out OK or ok or O.K. as its full word ‘okay’. But I wanted to question that.

My favourite style guide Chicago Manual of Style (US- used in publishing, academics, and more across countries), and The Greenslade Free Australian Style Guide (AUS), New Oxford Style Manual (UK), and The Canadian Press Stylebook (CAD) all do not indicate which to use.

Correction: The Canadian Press CAPS and SPELLING 20th ed. does indicate that OK is preferred. Thank you to the commenter who wrote in to correct this error.

When a style manual or guide doesn’t tell me what to use, I then rely on the dictionary for the type of English I am editing.

For US editing I predominantly use Merriam-Webster, for UK editing I use Oxford English Dictionary  and Cambridge Dictionary, and for CAD editing I use Oxford Canadian English Dictionary (paperback) and the Canadian Press’s spelling guide.

Here is what I discovered:

• US, UK dictionaries list OK as the most common spelling
• CAD dictionary lists Okay as the main spelling with the OK as a variant
• No one knows for sure where OK originated, but they think it was from a president’s nickname
• Okay is an informal derivative/spelling, but OK itself is considered informal language

So, if you’re wondering which is right… well, all of them, but OK is the most commonly used.