After changing it back and forth several times and consulting with internet linguist Gretchen McCulloch, I settled on "ok" in my book How To, but I'm still on the fence. Maybe I should just switch to "oK."

"OK/okay/ok", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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After changing it back and forth several times and consulting with internet linguist Gretchen McCulloch, I settled on "ok" in my book How To, but I'm still on the fence. Maybe I should just switch to "oK."