Glioneuronal tumors (GNTs) are rare neoplasms consisting of both glial and neuronal components. Among the GNTs, dysembryoplastic neuroepithelial tumors (DNTs), papillary glioneuronal tumors (PGNTs), and rosette-forming glioneuronal tumors of the fourth ventricle (RGNTs) share the character of being mainly composed of small round Olig2-positive tumor cells. Using immunohistochemistry and fluorescence in situ hybridization, we examined a series of 35 GNT cases (11 DNTs, 15 PGNTs and 9 RGNTs) on the characteristics of Olig2-positive tumor cells. Histologically, Olig2-positive cells showed small round forms in most GNTs; however, there were a small number of Olig2-positive cells with neuronal morphology only in a PGNT case. These cells expressed both glial and neuronal markers by double immunostaining. With regard to labeling indices and intensity, only PGNT cells expressed neuronal markers, including α-internexin and neurofilament. These findings also suggest that some Olig2-positive PGNT cells may show neuronal differentiation. In GNTs, a considerable number of Olig2-positive cells showed immunopositivity for cyclin D1 and/or platelet-derived growth factor receptor alpha (PDGFRα), which are markers for oligodendrocyte progenitor cells. These immunostainings were particularly strong in DNTs. In RGNTs, Olig2-positive cells formed “neurocytic rosettes”. Furthermore, they were also immunopositive for glial markers, including GFAP, PDGFRα and cyclin D1. These findings indicate the heterogeneous characteristics of Olig2-positive cells in GNTs, and some of them also exhibited neuronal features. So it is possible that a part of Olig2-positive GNT cells have characteristics similar to those of progenitor cells.