Oxygen

ChemGlobe: Oxygen - Electronic, thermal, and steric data along with basic information.
partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/8.html

ChemicalElements.com: Oxygen - Basic information, atomic structure, and table of isotopes.
www.chemicalelements.com/elements/o.html

EnvironmentalChemistry.com: Oxygen - Atomic structure, chemical and physical properties, and table of nuclides.
environmentalchemistry.com/yogi/periodic/O.html

Especiál Gas Inc.: Oxygen - Numeric data, uses, and general information.
www.c-f-c.com/specgas_products/oxygen.htm

It's Elemental: Oxygen - Basic physical and historical information.
education.jlab.org/itselemental/ele008.html

LANL: Oxygen - History, sources, forms, compounds, and isotopes.
periodic.lanl.gov/elements/8.html

Lenntech: Oxygen - Physical data, chemical properties, and health effects.
www.lenntech.com/Periodic-chart-elements/O-en.htm

Visual Elements: Oxygen - Animation, uses, physical information, and key isotopes.
www.chemsoc.org/viselements/pages/oxygen.html

WebElements: Oxygen - Information on history, uses, occurrence, compounds, and properties of the element.
www.webelements.com/webelements/elements/text/O/key.html

Wikipedia: Oxygen - Properties of the element, including its history, applications, and characteristics.
en.wikipedia.org/wiki/Oxygen