Whoa rhymes with woe. But while woe involves sadness, whoa means wow. While, "Wow!" is a fitting response to many events, it's pretty much never associated with sadness.

But where does whoa come from? Like many of the words we talk about here, whoa comes from Middle English. It's a variation of ho.

Given how close we are to Christmas, if you happen to see a rather stout old gentleman in red riding through the sky and calling "Ho, ho, ho!", you might suggest he update his patter and try, "Whoa!"

On that other hand, that might stop the reindeer before they reach a rooftop. Then you might want to use whoa's homonym after all.