Over the past decade, graphene oxide (GO), an oxidized derivative of graphene, has been extensively used in various fields such as water treatment, ion sieving, gas separation, drug delivery, sensing, and energy storage due to the presence of oxygen-containing functional groups, which provide GO with intrinsic hydrophilicity, dispersibility, superpermeability, and insulating properties. Furthermore, depending on the requirements of different applications, these groups can be modified by bonding with various organic functional groups to improve these characteristics. This chemical modification approach is known as functionalization, and can be divided into covalent and non-covalent depending on the bonding between the GO and the additive. Although both covalent functionalization and non-covalent functionalization can be used with GO, covalent functionalization has greater versatility in terms of the modification mechanisms and long-term stability. In this review, we focus on the covalent functionalization of GO and discuss four effective covalent functionalization methods, which are determined by the specific oxygen-containing functional groups. We explain their respective functionalization mechanisms and finally summarize the impact of covalent functionalization on GO and explore its future potential.