  The atmospheric C/O ratio of exoplanets is widely used to constrain their
formation. To guarantee that the C/O ratio provides robust information, we need
to accurately quantify the amount of C and O in exoplanetary atmospheres. In
the case of O, water and carbon monoxide are generally studied as the two key
carriers. However, oxygen is a very reactive element and does not bind with
carbon; depending on the temperature, it also binds to refractory elements.
Estimating the amount of oxygen bound to refractory elements is therefore
critical for unbiased estimates of the C/O ratio. In this work, we investigate
the oxygen deficit due to refractory elements and its effects on the
atmospheric C/O ratio of giant exoplanets as a function of their metallicity
and equilibrium temperature. We model the composition of planetary atmospheres
assuming chemical equilibrium and using as input physically justified elemental
mixtures arising from detailed planet formation simulations. Our results show
how the interplay between the atmospheric temperature and non-solar abundances
of oxygen and refractory elements can sequester large fractions of oxygen,
introducing significant biases in evaluating the C/O ratio when this effect is
not accounted for. We apply our results to the case of Jupiter in the Solar
System and show how the currently estimated water abundance points to a true
oxygen abundance that is four times the solar one.
