Oh I see - Uranyl Acetate is radioactive and this replaces it. Fun!

This seems like a friendly chemical too - “ The chemical properties of Osmium Tetroxide are such that use and handling of the chemical is often considered daunting. Although its volatility and toxicity certainly makes it a dangerous chemical, but when following the proper procedure and taking the necessary precautions, Osmium Tetroxide can be used to its full potential with limited risk to the user.

This is more toxic than glutaraldehyde and has a higher vapour pressure. Particular care must be taken to avoid breathing the vapour or allowing it to affect the eyes. ”