Introducing magnetic impurities onto a clean metal surface gives rise to the Kondo effect. If the substrate is superconducting, the impurities additionally create a pair-breaking potential for Cooper pairs, resulting in the so-called Yu-Shiba-Rusinov bound states. In a joint project with the group of Prof. Giorgio Sangiovanni we demonstrated that the YSR states of single Fe atoms on Nb(110) are strongly effected by the adsorbtion site of the atoms and the presence of oxygen on the surface. Oxygen increases the coupling of the impurity spin to the conduction electrons, leading to strong Kondo screening and a quantum phase transition of the YSR bound states. The result has been published in Phys. Rev. B.