How does NMO reform Osmium tetroxide after reaction with an alkene?
N-methyl-morpholine-N-oxide (NMO) is used to reoxidize and regenerate osmium tetroxide ($\ce{OsO4}$) in what is known as the "Upjohn Dihydroxylation" procedure.  

Mechanistically, the redox occurs when a lone pair of electrons on the reduced $\ce{Os(VI)O3}$ are used to form a bond to the morpholine N-oxygen; the electrons from the former $\ce{N-O}$ bond are, in turn, transferred to the morpholine nitrogen.  The $\ce{OsO3}$ gains an oxygen and is oxidized back to osmium (VIII) tetroxide and the NMO is reduced to N-methylmorpholine



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Osmium tetroxide is both toxic and expensive. Using the Upjohn procedure allows the reaction to be run with a catalytic amount of osmium teroxide and 1 equivalent of NMO.