The light alloying element in the Earth's core has not been identified yet. Here we determined the pressure-volume equations of state of FeSi, Fe3C, and Fe0.95O in the core pressure range by a combination of diamond-anvil cell and synchrotron X-ray diffraction techniques. Both B2-type FeSi and Fe3C cementite were preserved to 180 and 187 GPa, respectively. The rhombohedrally-distorted B1 phase of Fe0.95O was measured up to 186 GPa, and the distorted B8-type Fe0.95O was observed between 170 and 226 GPa. Combined with our previous data on FeS VI and B2-type VII phases to 270 GPa, we discuss the light element in the outer core by comparing the densities and compressibilities of these iron compounds with seismologically-estimated density profile in the core. Substitution of light element, particularly carbon and oxygen, in iron not only reduces the density but also enhances the compressibility remarkably. The core profile is therefore not reconciled with Fe-C and Fe-O compounds, while the densities and compressibilities of Fe-Si and Fe-S alloys match the observations. Carbon and oxygen may not be a predominant light element in the Earth's outer core, leaving silicon and sulfur as strong candidates.