A crystal-chemical exploration of part of the Li-Mn-N-O system was carried out. Several samples were synthesized using Li3N, MnxN and Li2O and characterized with chemical analysis, XRD, XAS, and NMR. An increase in the starting proportion of Li2O increases the amounts of lithium and oxygen in the compounds, but, according to the XANES Mn K-edge spectra, all the oxynitrides still contain Mn5+ ions preferentially coordinated by N3-, forming [MnN4] tetrahedra. The analysis of the position of these samples in the compositional Li3N-Li2O-MnN, ternary phase diagram and the plot of their cell parameters against the oxygen molar fraction indicates that all the oxynitrides belong to the same tieline, which also includes Li2O but not Li7MnN4. Although the XRD patterns suggest that these samples crystallize in a disordered antifluorite-type structure, the analysis of the Li-6 NMR data indicates that short-range ordering does exist. The performance as electrode materials in lithium batteries of the synthesized samples was also evaluated. Li7.9MnN3.2O1.6 was shown to be the most attractive candidate because of its higher capacity values and improved retention upon cycling with respect to the other members of the series.