With its low-energy extension DeepCore, the IceCube Neutrino Observatory, located at the geographic South Pole, measures neutrinos with energies above about 10 GeV. With this low energy threshold high statistics of about 150000 triggered atmospheric muon neutrinos are recorded per year. This enables the measurement of neutrino oscillations. The oscillation probability depends on the energy and propagation distance of the neutrino. The oscillation is visible by the energy and zenith dependent disappearance in the recorded muon neutrino rate. Recently, an analysis using three years of data of the completed 86-string detector taken from 2011 to 2014 has achieved a sensitivity approaching that of dedicated oscillation experiments (e.g. MINOS, T2K and Super-Kamiokande-IV). To further improve the sensitivity, this analysis has been extended with data taken from 2010 to 2011 in the 79-string detector configuration, which increases the live time from 953 to 1266 days. Results of this extended muon neutrino disappearance measurement with IceCube/DeepCore will be presented.