The design of new materials is a pivotal question for the development of the next generation of electrochemical energy storage and conversion devices. Recently, triggering the redox activity of oxygen ions in transition metal oxides used as positive electrodes for Li-ion batteries was proposed as a promising strategy to develop Li-rich compounds with reversible capacity exceeding those of classical insertion layered compounds. Nevertheless, the science at play must be rationalized in order to control this phenomenon and assess the potential applications of such Li-rich electrodes. Indeed, following this discovery, fundamental questions remain. Among them, the exact nature of the oxygen species formed upon oxidation is critical since it governs not only its redox potential but also the discharge behavior for which sluggish kinetics and hysteresis were observed. Using recent examples, the different types of oxygen species will be discussed and their electrochemical behavior compared. Moreover, while early measurements suggested gaseous oxygen to be produced for these Li-rich at potentials above 4.3 V vs. Li⁺/Li, recent observations point toward oxygen evolution within a potential range that can span from 3.5 to 4.5 V vs. Li⁺/Li. This observation asks for 1) the development of new analytical tools in order to accurately detect and quantify these gaseous species and 2) to rationalize the standard potential for the evolution of lattice oxygen from the transition metal oxides. These two points will be discussed and strategies to circumvent this detrimental phenomenon will be proposed.