In the iron(II)-thiolate models of cysteine dioxygenase, the thiolate ligand is a key factor in the oxygen activation. In this contribution, four model compounds have been theoretically investigated. This comparative study reveals that the thiolate ligand itself and its relative position are both important for the activation of O2. Before the O2 binding, the thiolate ligand must transfer charge to Fe(II), and the effective nuclear charges of Fe(II) is decreased, which results in a lower redox potential of compounds. In other words, the thiolate ligand provides a prerequisite for the O2 activation. Furthermore, the relative position of the thiolate ligand is discovered to determine the reaction path of O2 activation. The amount of charge transfer is crucial for these reactions, the more charge it transfers, the lower the related redox potentials are. This work really helps think deeper into the O2 activation process of mononuclear non-heme iron enzymes.