  Context: Dark cloud chemical models usually predict large amounts of O2,
often above observational limits. Aims: We investigate the reason for this
discrepancy from a theoretical point of view, inspired by the studies of
Jenkins and Whittet on oxygen depletion. Methods: We use the gas-grain code
Nautilus with an up-to-date gas-phase network to study the sensitivity of the
molecular oxygen abundance to the oxygen elemental abundance. We use the rate
coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic
Database for Astrochemistry) experts. Results: The updates of rate coefficients
and branching ratios of the reactions of our gas-phase chemical network,
especially N + CN and H3+ + O, have changed the model sensitivity to the oxygen
elemental abundance. In addition, the gas-phase abundances calculated with our
gas-grain model are less sensitive to the elemental C/O ratio than those
computed with a pure gas-phase model. The grain surface chemistry plays the
role of a buffer absorbing most of the extra carbon. Finally, to reproduce the
low abundance of molecular oxygen observed in dark clouds at all times, we need
an oxygen elemental abundance smaller than 1.6E-4. Conclusion: The chemistry of
molecular oxygen in dense clouds is quite sensitive to model parameters that
are not necessarily well constrained. That O2 abundance may be sensitive to
nitrogen chemistry is an indication of the complexity of interstellar
chemistry.
