This thesis deals mainly with two oxidation reactions: water oxidation and aerobic oxidation, both of which have been applied in a biomimetic fashion. In the former reaction molecular oxygen is generated whereas in the latter it was used as terminal oxidant in oxidation reactions. The first part of this thesis describes the synthesis of different ruthenium and manganese complexes that could potentially act as catalysts for water oxidation. This part includes a discussion of the stability and reactivity of a new manganese(III) amide-type complex, that has been used as a catalyst for both epoxidation of stilbene and alcohol oxidation. The second part of this thesis discusses the synthesis of two new hybrid catalysts consisting of hydroquinone linked cobalt(II) salophen and cobalt(II) salmdpt, which have been used as oxygen-activating catalysts in aerobic oxidation reactions. The former catalyst was applied to the Pd-catalyzed reactions such as 1,4-diacetoxylation of cyclohexadiene whereas the latter was applied to the Ru-catalyzed oxidation of secondary alcohols to ketones. Moreover, these two hybrid catalysts could be used in the Pd-catalyzed carbocyclization of enallenes. In all cases molecular oxygen was used as the stoichiometric oxidant.