Primary bone tumors, osteosarcomas and chondrosarcomas, derive from mesenchymal stem cells committed into osteoblasts and chondrocytes; in Ewing sarcomas (ESs), the oncogenic fusion protein EWS-FLI1 prevents mesenchymal differentiation and induces neuroectodermic features. Oncostatin M (OSM) is a cytokine from the IL-6 family that modulates proliferation and differentiation in numerous cells. The basis for inhibition versus induction of proliferation by this cytokine is obscure, although MYC was described as a potent molecular switch in OSM signaling. We show herein that, in contrast to osteosarcomas and chondrosarcomas, for which OSM was cytostatic, OSM induced proliferation of ES cell lines. Knockdown experiments demonstrated that growth induction by OSM depends on both types I [leukemia inhibitory factor receptor (LIFR)] and II [OSM receptor (OSMR)] receptors, high STAT3 activation, and induction of MYC to a high expression level. Indeed, ES cell lines, mice xenografts, and patient biopsy specimens poorly expressed LIF, precluding LIFR lysosomal degradation and OSMR transcriptional induction, thus leading to a high LIFR/OSMR ratio. Because other neuroectodermic tumors (ie, glioma, medulloblastoma, and neuroblastoma) had a similar expression profile, the main role of EWS-FLI1 could be through maintenance of stemness and neuroectodermic features, characterized by a low LIF, a high LIFR/OSMR ratio, and high MYC expression. Thus, this study on rare bone malignancies gives valuable insights on more common cancer regulatory mechanisms and could provide new therapeutic opportunities.