Source : Webster's Revised Unabridged Dictionary (1913)

Ora \O"ra\, n. [AS. See 2d {Ore}.]
   A money of account among the Anglo-Saxons, valued, in the
   Domesday Book, at twenty pence sterling.

Os \Os\, n.; pl. {Ora}. [L.]
   A mouth; an opening; an entrance.

Source : WordNet®

os
     n 1: a mouth or mouthlike opening
     2: a hard brittle blue-gray or blue-black metallic element that
        is one of the platinum metals; the heaviest metal known
        [syn: {osmium}, {atomic number 76}]
     3: (computer science) software that controls the execution of
        computer programs and may provide various services [syn: {operating
        system}]
     4: the left eye [syn: {oculus sinister}]
     5: rigid connective tissue that makes up the skeleton of
        vertebrates [syn: {bone}]
     [also: {ossa} (pl), {osar} (pl), {ora} (pl)]

ora
     See {os}