  We present the first statistically significant detection of neutrino
oscillations in the high-energy regime ($>$ 20 GeV) from an analysis of IceCube
Neutrino Observatory data collected in 2010-2011. This measurement is made
possible by the low energy threshold of the DeepCore detector ($\sim 20$ GeV)
and benefits from the use of the IceCube detector as a veto against cosmic
ray-induced muon background. The oscillation signal was detected within a
low-energy muon neutrino sample (20 -- 100 GeV) extracted from data collected
by DeepCore. A high-energy muon neutrino sample (100 GeV -- 10 TeV) was
extracted from IceCube data to constrain systematic uncertainties.
Disappearance of low-energy upward-going muon neutrinos was observed, and the
non-oscillation hypothesis is rejected with more than $5\sigma$ significance.
In a two-neutrino flavor formalism, our data are best described by the
atmospheric neutrino oscillation parameters $\Delta m^2_{23}=
(2.3^{+0.6}_{-0.5})\cdot 10^{-3}$ eV$^2$ and $\sin^2(2 \theta_{23})>0.93$, and
maximum mixing is favored.
