Answer: "whoa" or "woah"?

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Reader carodee wanted to know: Which is the correct spelling: whoa or woah?

I was pretty firmly in the camp of whoa when I got this question. My New Oxford American Dictionary doesn't list woah at all, and under whoa it says:

used as a command to a horse to make it stop or slow down
[informal] used as a greeting, to express surprise or interest, or to command attention

And the Corpus of Historical American English—which tracks language usage through magazines, newspapers, and fiction and nonfiction books—lists 911 uses of whoa in published sources versus ten of woah.

Seems pretty conclusive, right? ( Well, it wouldn't be a post if that was the end of the story. )