Constitutive equations that describe the hot flow stresses of commercial purity copper are strongly dependent on the quantity of oxygen. Even fire-refined coppers, which are 99.9% pure and contain considerable quantities of phosphorus, are heavily influenced by the precipitation of Cu2O rather than by the formation of other phases. The construction of a set equations that accurately describes the stress as a function of strain, strain rate and temperature is of the utmost importance when modeling hot working operations through finite element methods. Most of the high temperature stress-strain rate relationships account for the calculation of an activation energy. The proper selection of a so-called apparent activation energy Q(app) has been well accepted as a method to save the arduous work of analyzing additional strengthening mechanisms individually when the copper is affected by precipitation or by solid solution. Although Ravichandran et al. has described the variation of this apparent activation energy with the oxygen content, the present work adds new information concerning such relationship and analyses the proposed method using three commercial fire-refined coppers with different oxygen levels (26, 46, and, 62ppm).