We use atmospheric neutrino phenomenology to obtain the expected background to proton decay in large deep underground neutrino detector (DUNE)-like neutrino detectors. We introduced, for the first time in this kind of analysis, the experimentally confirmed neutrino oscillations of the atmospheric neutrino observations which reduce the corresponding background for the nucleon decay channel by a factor of 40%. Furthermore, we infer the impact of four systematics on such background: the overall efficiency, the muon reconstruction energy resolution, the resonant neutral pion cross-section, and the neutral pion angular resolution. Considering a 40 kton detector with efficiency of 45%, our analysis leads to an error band in the lower limit for the proton lifetime sensitivity, τ/B, from 7.9 × 1033 years to 1.1 × 1034 years at 90% confidence level (C.L.). These numbers can be compared with the current mode-dependent experimental limits τ > 1031 − 1033 years at 90% C.L.