[en] The structures, relative stabilities and electronic properties of Os_mC_n clusters for m+n≤7 have been investigated by the density functional theory. The planar geometries have been preferred for small carbon mixed clusters up to m+n=5, for those with size of m+n≥6, the C-rich clusters are still planar structures, while Os-rich clusters are three-dimensional structures, and the most energetic favourable structures tend to be capped pyramid or irregular prism. The stability analysis indicates that the mixed clusters are more stable than those of the corresponding pure Os_n clusters, among which OsC₂, Os₂C₂, Os₄C₂, Os₂C₄ exhibit higher stability than their neighbouring clusters. Os5d and C2p electrons play dominant roles in chemical reaction of mixed clusters. (author)