Context. Mass loss from massive stars, especially towards the end of their lives, plays a key role in their evolution. Radio emission from core-collapse supernovae (SNe) serves as a probe of the interaction of the SN ejecta with the circumstellar medium (CSM) and can reveal the mass-loss history of the progenitor. Aims. We aim to present broadband radio observations of the CSM-interacting SN 2019oys. SN 2019oys was first detected in the optical and was classified as a Type Ib SN. Then, ~100 days after discovery, it showed an optical rebrightening and a spectral transition to a spectrum dominated by strong narrow emission lines, which suggests strong interaction with a distant, dense, CSM shell. Methods. We modelled the broadband, multi-epoch radio spectra, covering 2.2 to 36 GHz and spanning from 22 to 1425 days after optical discovery, as a synchrotron emitting source. Using this modelling, we characterised the shockwave and the mass-loss rate of the progenitor. Results. Our broadband radio observations show strong synchrotron emission. This emission, as observed 201 and 221 days after optical discovery, exhibits signs of free-free absorption from the material in front of the shock travelling in the CSM. In addition, the steep power law of the optically thin regime points towards synchrotron cooling of the radiating electrons. Analysing these spectra in the context of the SN-CSM interaction model gives a shock velocity of 11 000 km s-1 (for a radius evolution of Δt0.8, where Δt is the time since optical discovery) and an electron number density of 4.1 × 105 cm-3 at a distance of 2.6 × 1016 cm. This translates to a high mass-loss rate from the progenitor massive star of 10-3 M⊙ yr-1 for an assumed wind of 100 km s-1 (assuming a constant mass-loss rate in steady winds). The late-time radio spectra, 392 and 557 days after optical discovery, show broad spectral peaks. We show that this can be explained by introducing a non-homogeneous CSM structure.