  The electronic transport of monolayer graphene devices is studied before and
after \emph{in situ} deposition of a sub-monolayer coating of osmium adatoms.
Unexpectedly, and unlike all other metallic adatoms studied to date, osmium
adatoms shift the charge neutrality point to more positive gate voltages. This
indicates that osmium adatoms act as electron acceptors and thus leave the
graphene hole-doped. Analysis of transport data suggest that Os adatoms behave
as charged impurity scatterers, albeit with a surprisingly low charge-doping
efficiency. The charge neutrality point of graphene is found to vary
non-monotonically with gate voltage as the sample is warmed to room
temperature, suggesting that osmium diffuses on the surface but is not
completely removed.
