History

(Gr. osme, a smell) Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia.

Sources

Osmium occurs in iridosule and in platinum-bearing river sands in the Urals, North America, and South America. It is also found in the nickel-bearing ores of Sudbury, Ontario region along with other platinum metals. While the quantity of platinum metals in these ores is very small, the large tonnages of processed nickel ores make commercial recovery possible.

Properties

The metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and the lowest vapor pressure of the platinum group. The metal is very difficult to fabricate, but the powdered or spongy metal slowly gives off osmium tetroxide, which as a powerful oxidizing agent and has a strong smell. The tetroxide is highly toxic, and boils at 130oC.
The measured densities of iridium and osmium seem to indicate that osmium is slightly more dense than iridium, so osmium has generally been credited with being the heavier element. Calculations of the density from the space lattice which may be more reliable for these elements than actual measurements, however, give a density of 22.65 for iridium compared to 22.661 for osmium. Despite this information, no decision has been made as to which is heavier. 
A Periodic Table of the Elements at Los Alamos National Laboratory