A comparative study on the room temperature ferromagnetism of Co doped ZnO and CeO2 bulk samples is presented. Co–ZnO system has been observed to switch between ferromagnetic and paramagnetic by controlling the donor defects, interstitial zinc: Zni. On the other hand, Co–CeO2 is always ferromagnetic. However, ferromagnetism increases/decreases with increase/decrease in donor defects, oxygen vacancies: VO. Systemic structural, magnetic, and transport analyses reveal that the nature of donor defects and host oxide plays a vital role in establishing ferromagnetism. This study provides an insight into the underlying mechanisms that are responsible for the ferromagnetism in Co–ZnO and Co–CeO2. Moreover, the proposed mechanisms are supported by the electronic structure of magnetic impurity ions and defects.