The Collaborative International Dictionary of English v.0.48:

Ho \Ho\, Hoa \Hoa\ (h[=o]), interj. [Cf. F. & G. ho.]
   1. Halloo! attend! -- a call to excite attention, or to give
      notice of approach. "What noise there, ho?" --Shak. "Ho!
      who's within?" --Shak.
      [1913 Webster]

   2. [Perhaps corrupted fr. hold; but cf. F. hau stop! and E.
      whoa.] Stop! stand still! hold! -- a word now used by
      teamsters, but formerly to order the cessation of
      anything. [Written also whoa, and, formerly, hoo.]
      [1913 Webster]

            The duke . . . pulled out his sword and cried "Hoo!"
                                                  --Chaucer.
      [1913 Webster]

            An herald on a scaffold made an hoo.  --Chaucer.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whoa \Whoa\, interj.
   Stop; stand; hold. See Ho, 2.
   [1913 Webster]