The catalytic activity of [CpOsBr2(PPh3)] in conjunction with Mg has been evaluated for atom transfer radical addition (ATRA) and cyclization (ATRC) reactions. The Os complex enabled these reactions to be performed with similar efficiency as that of the analogous Ru complex [CpRuCl2(PPh3)]. The olefin complex [CpOsBr(H2C=CHPh)(PPh3)] was obtained by reduction of [CpOsBr2(PPh3)] with Mg in the presence of an excess of styrene, whereas an analogous Ru complex was not observed. Kinetic investigations suggest that olefin complexes of Os can form under catalytic conditions.