The Collaborative International Dictionary of English v.0.48:

Hollo \Hol*lo"\, interj. & n. [See Halloo, and cf. Holla.]
   Ho there; stop; attend; hence, a loud cry or a call to
   attract attention; a halloo.
   [1913 Webster]

         And every day, for food or play,
         Came to the mariner's hollo.             --Coleridge.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hollo \Hol"lo\, v. i. [imp. & p. p. Holloed; p. pr. & vb. n.
   Holloing.] [See Hollo, interj., and cf. Halloo.]
   To call out or exclaim; to halloo.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hollo
    n 1: a very loud utterance (like the sound of an animal); "his
         bellow filled the hallway" [syn: bellow, bellowing,
         holla, holler, hollering, hollo, holloa, roar,
         roaring, yowl]
    v 1: encourage somebody by crying hollo
    2: cry hollo
    3: utter a sudden loud cry; "she cried with pain when the doctor
       inserted the needle"; "I yelled to her from the window but
       she couldn't hear me" [syn: shout, shout out, cry,
       call, yell, scream, holler, hollo, squall]