The Collaborative International Dictionary of English v.0.48:

Ox \Ox\ ([o^]ks), n.; pl. Oxen. [AS. oxa; akin to D. os. G.
   ochs, ochse, OHG. ohso, Icel. oxi, Sw. & Dan. oxe, Goth.
   a['u]hsa, Skr. ukshan ox, bull; cf. Skr. uksh to sprinkle.
   [root]214. Cf. Humid, Aurochs.] (Zool.)
   The male of bovine quadrupeds, especially the domestic animal
   when castrated and grown to its full size, or nearly so. The
   word is also applied, as a general name, to any species of
   bovine animals, male and female.
   [1913 Webster]

         All sheep and oxen, yea, and the beasts of the field.
                                                  --Ps. viii. 7.
   [1913 Webster]

   Note: The castrated male is called a steer until it attains
         its full growth, and then, an ox; but if castrated
         somewhat late in life, it is called a stag. The male,
         not castrated, is called a bull. These distinctions are
         well established in regard to domestic animals of this
         genus. When wild animals of this kind are spoken of, ox
         is often applied both to the male and the female. The
         name ox is never applied to the individual cow, or
         female, of the domestic kind. Oxen may comprehend both
         the male and the female.
         [1913 Webster]

   Grunting ox (Zool.), the yak.

   Indian ox (Zool.), the zebu.

   Javan ox (Zool.), the banteng.

   Musk ox. (Zool.) See under Musk.

   Ox bile. See Ox gall, below.

   Ox gall, the fresh gall of the domestic ox; -- used in the
      arts and in medicine.

   Ox pith, ox marrow. [Obs.] --Marston.

   Ox ray (Zool.), a very large ray (Dicerobatis Giornae) of
      Southern Europe. It has a hornlike organ projecting
      forward from each pectoral fin. It sometimes becomes
      twenty feet long and twenty-eight feet broad, and weighs
      over a ton. Called also sea devil.

   To have the black ox tread on one's foot, to be
      unfortunate; to know what sorrow is (because black oxen
      were sacrificed to Pluto). --Leigh Hunt.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oxen
    n 1: domesticated bovine animals as a group regardless of sex or
         age; "so many head of cattle"; "wait till the cows come
         home"; "seven thin and ill-favored kine"- Bible; "a team of
         oxen" [syn: cattle, cows, kine, oxen, Bos taurus]