The episode title is the chemical symbol for the element Osmium, the densest of the naturally occurring elements.
The glyphs for this episode spell out: EARTH
The Observer passes through the background at the crime scene outside the metal depository.
Security guard Kevin (uncredited Jorge Garcia) also plays Hurley on LOST, another show by JJ Abrams.
Astrid buys mints for Walter, who then drops one into a two-liter bottle of Coke, causing a geyser. This was a nod to the 2005 fad involving Mentos mints. The foaming was not a chemical reaction as is implied here, but was due to the micro-texture of the surface of the mint, which caused an instantaneous release of CO2 - the carbonization in the soft drink. This trick could be done with semi-polished pebbles and other objects.