spelling trivia of the day

The sound which one makes when one wishes a horse to stop, and, by extension, when one is surprised and wishes one's interlocutor to pause in conversation, is, in standard English, spelled "Whoa." Those with archaic sensibilities may prefer "Woa" or even "Wo." However, "Woah" is not standard and appears in the OED only as an example of usage (in 1856) under the heading "Woa."