Multiconfiguration Hartree-Fock (MCHF) and configuration interaction (CI) calculations have been performed for the low-lying levels 2p43P, 2p3(4S0)3s3,5S0and 2p3(4S0)3p3,5P0of neutral oxygen. Different cancellation effects on the specific mass shift, on the electronic contributions to hyperfine interaction constants and on the length and velocity forms of oscillator strengths are illustrated. The final CI values are compared with experimental values and, when available, with values from other theories.