A highly conductive and transparent indium tin oxide (ITO) film was developed using a nanoparticle-based solution process through the control of oxygen partial pressure during annealing. At an oxygen partial pressure of 2.1 × 10⁻³ Torr, a maximum conductivity of 313 Ω⁻¹ cm⁻¹ was obtained: a great improvement over the conductivity of conventional ITO nanoparticle films (at this conductivity, the sheet resistance decreased to 30 Ω sq⁻¹, and the transmittance reached 90%). By analyzing the electron concentration and mobility using Hall measurements, we determined that the main factor contributing to the enhanced conductivity is the increase in electron concentration that occurs due to the formation of oxygen vacancies under low oxygen partial pressures. However, if the oxygen partial pressure is too low, the removal of the organic ligands covering the ITO nanoparticles is incomplete, and the electron mobility is reduced. Microstructure control is also necessary for further improvement of the mobility.