A library of 29 organoosmium compounds has been built up with known and novel cyclometalated compounds obtained with C-N, N boolean AND C boolean AND N, and C boolean AND N boolean AND N ligands. All compounds have been tested for their in vitro cytotoxic properties against A172, a tumor cell line derived from a human glioblastoma, this affording a contrasted picture of the activities of the compounds gathered in this study. Some compounds displayed good to excellent activities, some of them showing IC50 in the nanomolar range. The level of activity was tentatively correlated to several physicochemical properties of the compounds such as their E-1/2(0)(Os-III/II) redox potential and their lipophilicity (log P-o/w). A parallel with related ruthenium derivatives was tentatively proposed.