We report experimental evidence for a microscopic mechanism influencing the family of morphologies that can be observed in thin-layer electrodeposition of metals. We show for different cations (zinc, copper and tin) that small quantities of oxygen, dissolved in the electrolyte, can modify dramatically the interfacial reduction mechanism and lead to drastic changes in the growth dynamics. We concentrate on zinc electrodeposition, and propose a phenomenological interpretation for the transition from dendrites to dense patterns.