  Recent results show that atmospheric $\nu_\mu$ oscillate with $\delta m^2
\simeq 3 \times 10^{-3}$ eV$^2$ and $\sin^2{2\theta_{atm}} \simeq 1$, and that
conversion into $\nu_e$ is strongly disfavored. The Super-Kamiokande (SK)
collaboration, using a combination of three techniques, reports that their data
favor $\nu_\mu \to \nu_\tau$ over $\nu_\mu \to \nu_{sterile}$. This distinction
is extremely important for both four-neutrino models and cosmology. We propose
that neutrino-proton elastic scattering ($\nu + p \to \nu + p$) in water
\v{C}erenkov detectors can also distinguish between active and sterile
oscillations. This was not previously recognized as a useful channel since only
about 2% of struck protons are above the \v{C}erenkov threshold. Nevertheless,
in the present SK data there should be about 40 identifiable events. We show
that these events have unique particle identification characteristics, point in
the direction of the incoming neutrinos, and correspond to a narrow range of
neutrino energies (1-3 GeV, oscillating near the horizon). This channel will be
particularly important in Hyper-Kamiokande, with $\sim 40$ times higher rate.
Our results have other important applications. First, for a similarly small
fraction of atmospheric neutrino quasielastic events, the proton is
relativistic. This uniquely selects $\nu_\mu$ (not $\bar{\nu}_\mu$) events,
useful for understanding matter effects, and allows determination of the
neutrino energy and direction, useful for the $L/E$ dependence of oscillations.
Second, using accelerator neutrinos, both elastic and quasielastic events with
relativistic protons can be seen in the K2K 1-kton near detector and MiniBooNE.
