Abstract: The effects of oxygen on the grain size, electrical conductivity and mechanical properties of 7N pure copper were investigated by adding different levels of oxygen to the copper, using electrical conductivity measurements, tensile tests and microstructure analysis. The presence of oxygen and the mechanism affecting the properties were analyzed. The results show that oxygen can effectively refine the grain size of pure copper in both cast and the hot-extruded states. When the oxygen content is 2400×10^-6, the hot extruded grain size can be refined from 94.307 μm to 17.76 μm. The reduction of the conductivity of pure copper by oxygen element is relatively small, and the conductivity is still above 100% IACS at an oxygen content of 410×10^-6. The effect of oxygen element on the tensile strength of pure copper is small, but it significantly reduces the plasticity. When the oxygen content is 410×10^-6, the elongation of pure copper can be reduced from 44.774% to 24.917%. Oxygen in pure copper mainly exists in the form of Cu₂O phase, which is distributed at the grain boundaries and in the grain interior. Cu₂O phase and copper matrix show a non-coherent interface relationship, with an interface mismatch of 27.7%, and a poor interface binding force, which leads to the reduction of plastic deformation ability.