The Collaborative International Dictionary of English v.0.48:

Oxygen \Ox"y*gen\, n. [F. oxyg[`e]ne, from Gr. 'oxy`s sharp,
   acid + root of gi`gnesqai to be born. So called because
   originally supposed to be an essential part of every acid.]
   [1913 Webster]
   1. (Chem.) A colorless, tasteless, odorless, gaseous element
      of atomic number 8, occurring in the free state in the
      atmosphere, of which it forms about 23 per cent by weight
      and about 21 per cent by volume, being slightly heavier
      than nitrogen. Symbol O. Atomic weight 15.9994.
      [1913 Webster +PJC]

   Note: It occurs combined in immense quantities, forming eight
         ninths by weight of water, and probably one half by
         weight of the entire solid crust of the globe, being an
         ingredient of silica, the silicates, sulphates,
         carbonates, nitrates, etc. Oxygen combines with all
         elements (except fluorine), forming oxides, bases,
         oxyacid anhydrides, etc., the process in general being
         called oxidation, of which combustion is only an
         intense modification. At ordinary temperatures with
         most substances it is moderately active, but at higher
         temperatures it is one of the most violent and powerful
         chemical agents known. It is indispensable in
         respiration, and in general is the most universally
         active and efficient element. It may be prepared in the
         pure state by heating potassium chlorate.
         [1913 Webster] This element (called dephlogisticated
         air by Priestley) was named oxygen by Lavoisier because
         he supposed it to be a constituent of all acids. This
         is not so in the case of a very few acids (as
         hydrochloric, hydrobromic, hydric sulphide, etc.), but
         these do contain elements analogous to oxygen in
         property and action. Moreover, the fact that most
         elements approach the nearer to acid qualities in
         proportion as they are combined with more oxygen, shows
         the great accuracy and breadth of Lavoisier's
         conception of its nature.
         [1913 Webster]

   2. Chlorine used in bleaching. [Manufacturing name]
      [1913 Webster]

WordNet (r) 3.0 (2006):

oxygen
    n 1: a nonmetallic bivalent element that is normally a colorless
         odorless tasteless nonflammable diatomic gas; constitutes
         21 percent of the atmosphere by volume; the most abundant
         element in the earth's crust [syn: oxygen, O, atomic
         number 8]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "oxygen":
   acetylene, ammonia, argon, asphyxiating gas, butane,
   carbon dioxide, carbon monoxide, chlorine, coal gas, ethane, ether,
   ethylene, fluorine, formaldehyde, helium, hydrogen,
   illuminating gas, krypton, lewisite, marsh gas, methane,
   mustard gas, natural gas, neon, nitrogen, ozone, poison gas,
   propane, radon, sewer gas, xenon



The Elements (07Nov00):

oxygen
Symbol: O
Atomic number: 8
Atomic weight: 15.9994
A colourless, odourless gaseous element belonging to group 16 of the
periodic table. It is the most abundant element present in the earth's
crust. It also makes up 20.8% of the Earth's atmosphere. For industrial
purposes, it is separated from liquid air by fractional distillation. It
is used in high temperature welding, and in breathing. It commonly comes
in the form of Oxygen, but is found as Ozone in the upper atmosphere. It
was discovered by Priestley in 1774.