  We study a scenario where sterile neutrino (either warm or cold) dark matter
(DM) is produced through (nonresonant) oscillations among right-handed
neutrinos (RHNs) and can constitute the whole DM in the Universe, in contrast
to the conventional sterile neutrino production through its mixing with the
left-handed neutrinos. The lightest RHN can be sterile neutrino DM whose mixing
with left-handed neutrinos is sufficiently small while heavier RHNs can have
non-negligible mixings with left-handed neutrinos to explain the neutrino
masses by the seesaw mechanism. We also demonstrate that, in our scenario, the
production of sterile RHN DM from the decay of a heavier RHN is subdominant
compared with the RHN oscillation production due to the X-ray and small-scale
structure constraints.
