Abstract: We investigate the temporal and spatiotemporal buoyancy instabilities in a horizontal liquid layer supported by a poorly conducting substrate and subjected to an imposed oblique temperature gradient (OTG) with the horizontal and vertical components, denoted as HTG and VTG, respectively. The general linear stability analysis (GLSA) reveals a strong stabilizing effect of the HTG on the instabilities introduced by the VTG for Prandtl numbers Pr >1 via inducing an extra vertical temperature gradient opposing the VTG through the energy convection. For Pr