We study the charge state of the diffusing O-2 molecule during silicon oxidation through hybrid functional calculations. We calculate charge-transition levels of O-2 in bulk SiO2 and use theoretical band offsets to align these levels with respect to the Si band edges. To overcome the band-gap problem of semilocal density functionals, we employ hybrid functionals with both predefined and empirically adjusted mixing coefficients. We find that the charge-transition level epsilon(0/-) in bulk SiO2 occurs at similar to 1.1 eV above the silicon conduction-band edge, implying that the O-2 molecule diffuses through the oxide in the neutral charge state. While interfacial effects concur to lower the charge-transition level, our estimates suggest that the neutral charge state persists until silicon oxidation.