  We present the first detailed simulation study of tau neutrino-induced
charged current (CC) events from atmospheric neutrino interactions in the Iron
Calorimeter (ICAL) detector at the proposed India-based Neutrino Observatory
(INO) laboratory. Since the intrinsic atmospheric neutrino flux at few to 10s
of GeV energy comprises only electron and muon neutrinos (and anti-neutrinos)
with negligible tau neutrino component, any signature of atmospheric tau
neutrinos is a signal for neutrino oscillations. We study the tau leptons
produced through these CC interactions via their hadronic decay. These events
appear as an excess over the neutral current (NC) background where hadrons are
the only observable component. We find that the presence of tau neutrinos in
the atmospheric neutrino flux can be demonstrated to nearly $4\sigma$
confidence with 10 years data; in addition, these events are sensitive to the
neutrino oscillation parameters, $\sin^2\theta_{23}$ and $\vert \Delta m_{31}^2
\vert$ (or $\vert \Delta m_{32}^2 \vert$), in the 2--3 sector. Finally, we show
that combining these events with the standard muon analysis which is the core
goal of ICAL further improves the precision with which these parameters,
especially the octant of $\theta_{23}$, can be measured.
