Then I read an article in The Guardian's review section (which I now can't find, so here's a link to an earlier article in the Guardian--by the outgoing poet laureate, no less) that contained a woah. As has been mentioned here before, The Guardian (or The Grauniad) has something of a reputation for bad spelling and typographical errors, so I remarked to myself that BrE writers seem to have a hard time spelling whoa.
Then I was in an English airport and I saw an ad(vert) (I wish I'd taken a photo, but I was too airport-grumpy at the time to think of it--it might've been for Phones4You), that shouted WOAH! WOAH! WOAH! in red and white. At that point, I had to start planning my admission of wrongness to BH.
(I'm sure many halves of long-standing and happy couples are thinking that I did not have to admit that I was wrong. Since BH neither saw the ad(vert) nor remembered the time I corrected him, what was to be gained by interfering with the well-developed roles of She Who Is Right and He Who Must Be Corrected? But, you see, I had to admit I'd been wrong because I have in the past claimed that admitting-when-I'm-wrong is something that I am happy to do, and so in order to prove myself right I have to prove myself wrong--on a regular basis.)
So, my story of whoa (and woah):
The OED lists woah as a variant of woa which is a variant of whoa, which is a variant of the interjection who (not to be confused with the pronoun who--the interjection is pronounced as wo--which is also a variant of all these), which came into the language as a variant of ho! Here are the dates of the OED's quotations for these spellings of the pronunciation /wo/ when it means 'stop!':
who c.1450-1859It's interesting that the OED lists woa as a variant of whoa when it has earlier evidence for woa--it implies that whoa is the more standard form. We shouldn't read much into the lack of recent examples of any of these--it looks like nothing has been added to these entries since the first edition.
woah 1856 (one example--included under the headword woa)
whoa 1843-1898 (but, of course, we know it's still used)
I don't remember ever seeing the woah spelling (I'd want to pronounce it as two syllables: wo-ah, like Noah) before moving to England, but it's a very popular spelling here. Searching just UK sites, one gets ~170,000 hits for woah and ~255,000 for whoa. Searching some American sites, one gets 33 woahs to 461 whoas on .mil and 8,800 to 39,000 on .edu (the first woahs that came up on the .edu search were quoted from a BBC site, though). Or, if you'd like to see some bar graphs showing US and UK usage of the spellings, try this.
(Can you believe I started this post on the 6th of April? Alas and alack--I wish I had a solid month to do nothing but catch up on this blog.)