This talk is about the occurrence of mixed-mode oscillations in Olsen model for the peroxidase-oxidase reaction. The model is studied using geometric-singular perturbation theory. It is four dimensional, contains two small parameters and is a three-time scale system, with one fast, two medium and one slow variable. I used the slowest variable as the only slow variable and the other three as fast variables. The small amplitude oscillations in the model arise due to a dynamic Hopf-bifurcation. The oscillations are organised by canard-like orbits (crossings of stable and unstable manifolds of saddle-slow manifolds). Upon parameter change, an arithmetic progression of mixed-mode oscillations is obtained. This can be readily explained by the canard-like orbits. In the second part of the talk, I compare my results, with the results by Kühn and Szmolyan, who analysed the Olsen model using only the fastest variable as the fast-variable and the other three as slow variables. Combining both analysis gives insights about the double limit, where both parameters are small.