The valence photoelectron spectrum (bands from A to E) of OsO4 is studied by second-order complete active space perturbation theory (CASPT2). The relativistic effects are included by the relativistic elimination of the small components (RESC) scheme. Spin-orbit coupling is also considered. Both the electron correlation and relativistic effects are significant on the peak position and intensity of valence photoelectron spectrum of OsO4. RESC-CASPT2 gives values for the peak position and intensity in reasonable agreement with the resolved photoelectron spectrum data. The state ordering is predicted as T-2(1)(1t(1)) > T-2(2)(3t(2)) > (2)A(1)(2a(1)) > T-2(2)(2t(2)) > E-2(2)(1e). Electron correlation shifts all peak positions by 2.5-3.5 eV to the high-energy side. Spin-free relativistic effects shift the band D by 0.8 eV again to the high-energy side due to the stabilization of Os 6s orbital. The present study confirms that the band C arises from the spin-orbit coupling in the ionic T-2(2)(3t(2)) state. Under the E band, there are many two-electron shake-up peaks in addition to the two one-electron ionization peaks. The broad feature of the E band is due to the satellite peaks and can be well explained through the introduction of the relativistic effects.