Sintered copper nanoparticles are being considered as alternatives to solder and/or sintered silver in different applications. Like for the alternatives, the interpretation of accelerated fatigue test results does however require modeling, typically involving prediction of stresses and strains versus time and temperature based on constitutive relations. This poses a challenge as the inelastic deformation properties depend strongly on both the initial particles and details of the processing, i.e., unlike for solder general constitutive relations are not possible. This work provides a mechanistic description of the early transient creep of relevance in cycling, including effects of sintering parameters and subsequent oxidation. Inelastic deformation is dominated by diffusion, rather than dislocation motion. Generalized constitutive relations are provided to the extent that quantitative modeling of a specific structure only requires the measurement of a single creep curve for that.