  The ongoing and future searches for diffuse supernova neutrinos and sterile
neutrinos carried out with large water-Cherenkov detectors require a precise
determination of the backgrounds, especially those involving gamma rays. Of
great importance, in this context, is the process of neutron knockout through
neutral-current scattering of atmospheric neutrinos on oxygen. Nuclear
reinteractions of the produced neutron may in fact lead to the production of
gamma rays of energies high enough to mimic the processes of interest. In this
Letter, we focus on the kinematical range suitable for simulations of
atmospheric-neutrino interactions and provide the neutron-knockout cross
sections computed using the formalism based on realistic nuclear spectral
function. The role of the strange-quark contribution to the neutral-current
axial form factor is also analyzed. Based on the available experimental
information, we give an estimate of the associated uncertainty.
