Tom Skerritt. Biography: Tom Skerritt is probably the best-known actor whose name is never remembered. A rugged outdoors type, Skerritt briefly attended Wayne State University and UCLA before making his film bow in [[Feature~V115961~War Hunt~warhunt]] (1962). His...
The chemical formula for oxygen is O2 and for individual oxygen  atom its symbol is O.   Gases such as oxygen exist as diatomic molecules.
The chemical formula for water is H 2 O which means that every molecule of water has 2 atoms of hydrogen (H) and one atom of oxygen (O). Here comes the key part. From the Periodic Table of Elements, one sees that one mole of hydrogen atoms weighs 1 gram while one mole of oxygen atoms weighs 16 grams.