We present a study of the three forbidden oxygen lines [OI] located in the optical region (i.e., 5577.339 r{A} (the green line), 6300.304 r{A} and 6363.776 r{A} (the two red lines)) in order to better understand the production of these atoms in cometary atmospheres. The analysis is based on 48 high-resolution and high signal-to-noise spectra collected with UVES at the ESO VLT between 2003 and 2011 referring to 12 comets of different origins observed at various heliocentric distances. The flux ratio of the green line to the sum of the two red lines is evaluated to determine the parent species of the oxygen atoms by comparison with theoretical models. This analysis confirms that, at about 1 AU, H2O is the main parent molecule producing oxygen atoms. At heliocentric distances > 2.5 AU, this ratio is changing rapidly, an indication that other molecules are starting to contribute. CO and CO2, the most abundant species after H2O in the coma, are good candidates and the ratio is used to estimate their abundances. We found that the CO2 abundance relative to H2O in comet C/2001 Q4 (NEAT) observed at 4 AU can be as high as ~70 %. The intrinsic widths of the oxygen lines were also measured. The green line is on average about 1 km/s broader than the red lines while the theory predicts the red lines to be broader. This might be due to the nature of the excitation source and/or a contribution of CO2 as parent molecule of the 5577.339 r{A} line. At 4 AU, we found that the width of the green and red lines in comet C/2001 Q4 are the same which could be explained if CO2 becomes the main contributor for the three [OI] lines at high heliocentric distances.
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