ChemGlobe: Osmium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Osmium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Osmium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Osmium: Basic physical and historical information.
LANL: Osmium: Sources, properties, handling, and uses.
Lenntech: Osmium: Physical data, chemical properties, health and environmental effects.
Osmium: Data tables and historic information.
Radiochemistry of Osmium: Full text of the monograph by G. W. Leddicotte (Oak Ridge National Laboratory, Oak Ridge, Tennessee). [PDF]
Visual Elements: Osmium: General and physical information, source, uses, key isotopes, and ionisation energies.
WebElements: Osmium: Extensive information on history, uses, occurrence, compounds, and properties.
Wikipedia: Osmium: Properties of the element, including its history, applications, and characteristics.