Yes, "wo" appears in major English dictionaries, though its status and usage vary. In most comprehensive dictionaries, "wo" is listed as an archaic or dialectal exclamation used to command a horse to stop or stand still—similar to "whoa." It's the variant spelling that predates the more modern "whoa" that's commonly used today.
Beyond this historical usage, "wo" has limited recognition as a standalone English word in standard dictionaries. Some dictionaries may include it as a variant or obsolete form, while others focus primarily on "whoa" as the standard form. In Scrabble dictionaries (like the Official Scrabble Players Dictionary), "wo" is typically accepted as a valid word, making it playable in word games.
The word's presence in dictionaries reflects English's evolution—older or regional terms sometimes persist in comprehensive references even when they're rarely used in modern speech or writing. If you're checking a specific dictionary, the inclusion and definition of "wo" may depend on how extensive that dictionary is and whether it includes archaic or variant forms. For practical purposes, "whoa" is the standard modern spelling, but "wo" remains a legitimate dictionary entry with historical credentials.