ChemGlobe: Oxygen: Electronic, thermal, and steric data along with basic information.
ChemicalElements.com: Oxygen: Basic information, atomic structure, and table of isotopes.
EnvironmentalChemistry.com: Oxygen: Atomic structure, chemical and physical properties, and table of nuclides.
Especiál Gas Inc.: Oxygen: Numeric data, uses, and general information.
It's Elemental: Oxygen: Basic physical and historical information.
LANL: Oxygen: History, sources, forms, compounds, and isotopes.
Lenntech: Oxygen: Physical data, chemical properties, and health effects.
Oxygen: Data tables and historic information.
Visual Elements: Oxygen: Animation, uses, physical information, and key isotopes.
WebElements: Oxygen: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Oxygen: Properties of the element, including its history, applications, and characteristics.