We presented a state-of-the-art simulation of the 1s O ionization of liquid water. We employed highly accurate EOM-CC methods adapted to core-level states by using the CVS scheme. Equilibrium sampling was carried out using MD and AIMD simulations. We carefully analyzed the effect of the embedding on the computed IEs and show that the convergence of the result with respect to the size of the QM system is slow. Our productionlevel calculations were carried out using the QM system of 20 water molecules embedded in the MM charges. We also evaluated the effect of triple excitations using the MLCC3 framework. Our calculations yield the value of the bulk IE of 538.75 eV and the FWHM of the bulk peak of 1.44 eV; the computed shift relative to the gas-phase IE is -1.51 eV (including MLCC3 and basis set corrections).