Whoa or Woah?

I don't know how the misspelling started, but a 2004 entry for woah on Urban Dictionary proves that it's at least a decade old, and a Google Ngram search shows much older occasional misspellings.

Here's a way to remember the proper spelling: Who and ho are two origins that are often cited for whoa. For example, Dictionary.com states that ho came first as a Middle English command to make a horse stop and then evolved sometime around 1620 into whoa. To remember how to spell whoa, remember that the original word—ho, something you'd say to a horse—stays intact in the middle.

Also, in 1616 in the play "The Winter's Tale," William Shakespeare had a character known only as Shepherd call out "Whoa-ho-hoa!" Say it in your mind: whoa-ho-hoa. Say it loudly like Santa Claus: whoa-ho-hoa! The extra hos after the whoa should also help you remember to keep the ho in the middle.