Laser-induced breakdown (LIB) spectroscopy in oxygen at room temperature and pressures ranging from 4.6 to 75 kPa was studied using a high-power transverse excitation atmospheric CO 2 laser (λ = 9.621 and 10.591 µm; τ FWHM = 64 ns; power densities ranging from 0.87 to 6.31 GW cm −2 ). The spectrum of the generated plasma is dominated by emission of strong O, O + and weak O 2+ atomic lines. Excitation temperatures of 31 500 ± 1600 K and 23 000 ± 3000 K were estimated by means of O 2+ and O + ionic lines, respectively. Electron number densities of the order of (3.5-16.5) × 10 16 cm −3 were deduced from the Stark broadening of several ionic O + lines. The characteristics of the spectral emission intensities from different species have been investigated as functions of the oxygen pressure and laser irradiance. Optical breakdown threshold intensities in O 2 at 10.591 µm have been determined. The physical processes leading to LIB of oxygen have been analysed.