Decreasing the nucleation density of graphene grown on copper (Cu) foil by chemical vapor deposition (CVD) is essential for the synthesis of large-area single-crystalline graphene. Here, the behavior of the copper oxide layer and its impact on the graphene growth have been investigated. We found that a small amount of oxygen dissolves into the Cu when the oxide layer decomposes during the heating up in a non reducing Ar environment. The remaining oxygen in the Cu foil can play an important role in decreasing the graphene nucleation density. The dissolved oxygen can withstand at high temperatures even in reducing H-2 environments without completely losing its effectiveness for maintaining a low graphene nucleation density. However, heating up in a H-2 environment significantly reduces the copper oxide layer during the very first moments of the process at low temperatures, preventing the oxygen to dissolve into the Cu and significantly increasing the nucleation density. These findings will help to improve the graphene growth on Cu catalyst by increasing the grain size while decreasing the grain density. (C) 2017 Elsevier B.V. All rights reserved.