Stoichiometric orthorhombic O-LiMnO2, prepared at high temperature from a mixture of Mn2O3 and LiOH.H2O, is electrochemically active in a lithium deintercalation/intercalation process. The samples with the smallest grains (O < 1 mu m), and corresponding to the phases prepared with some lithium hydroxide deficiency developed the higher weight capacities. These could reach up to 160 mAh/g under a C/15 regime following a forming that spanned over about thirty cycles. The cells capacity remained constant after forming, showing the very good cyclability of the system. It has not been clearly demonstrated whether the Li/Mn substitution in the O-LiMnO2 samples played an important role in the electrochemical characteristics of the Li/LixMnO2 cells, although the group with small grains also showed, on average, a higher cationic disorder, A phase transition took place upon the first oxidation of O-LiMnO2. The new phase presented an electrochemical behavior resembling that of spinel LiMn2O4. A modeling of the discharge curves showed a progressive forming of the materials during a few cycles.