  We present a series of near-infrared spectra of Nova Ophiuchus 2017 in the
$K$ band that record the evolution of the first overtone CO emission in
unprecedented detail. Starting from 11.7d after maximum, when CO is first
detected at great strength, the spectra track the CO emission to +25.6d by
which time it is found to have rapidly declined in strength by almost a factor
of $\sim$ 35. The cause for the rapid destruction of CO is examined in the
framework of different mechanisms for CO destruction viz. an increase in
photoionizating flux, chemical pathways of destruction or destruction by
energetic non-thermal particles created in shocks. From LTE modelling of the CO
emission, the $^{12}$C/$^{13}$C ratio is determined to be 1.6 $\pm$ 0.3. This
is consistent with the expected value of this parameter from nucleosynthesis
theory for a nova eruption occuring on a low mass ($\sim$ 0.6 M$_\odot$)
carbon-oxygen core white dwarf. The present $^{12}$C/$^{13}$C estimate
constitutes one of the most secure estimates of this ratio in a classical nova.
