Cation disorder is a phenomenon that is becoming increasingly important for the design of high-energy lithium transition-metal oxide positive electrodes (LiMO$_2$) for Li-ion batteries. Disordered Li-excess rocksalts have recently been shown to achieve high reversible capacity, and in operando cation disorder (i.e., disorder induced by electrochemical cycling) has been observed in a large class of ordered materials. Despite the growing importance of cation disorder in the Li-ion battery field, very little is known about the effect of cation disorder on the average voltage (i.e., energy density) of lithium transition metal oxides. In this study, we use first-principles methods to demonstrate that, depending on the transition metal species, cation disorder can lead to an increase or a decrease of the average voltage of lithium transition metal oxides. We further demonstrate that the Ni$^{3+/4+}$ redox can be high in disordered compounds, so that it may be preceded by oxygen activity. Finally, we establish rules for the voltage evolution of compounds that experience in operando disorder.