ONIX: An open-source depletion code

•New open-source depletion software for nuclear reactor physics simulations.•Provides coupling with open-source transport code OpenMC as well as standalone capability.•Provides developer-friendly Python interface, highly flexible nuclear data libraries processing, and output data postprocessing.•First depletion software adapted for nuclear archaeology and nuclear forensic analysis.•Software validated through comparisons with various numerical and experimental benchmarks.