We present a Li-7 MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O-2 layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres): aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO, cores (volume similar to 58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume similar to 20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-like regions, most probably surrounding the Li2MnO3 domains and thus labeled "Ni-poor boundaries" (volume similar to 21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (<= 4.3 V), Li-ion extraction occurs mainly from the (Ni-rich) Li(1-x)MO(2-)like domains via Ni2+ oxidation. At higher states of charge (>= 4.5 V), the Li2MnO3-like domains become highly involved via oxygen-based (ir)reversible oxidation processes, leading to significant structural transformations. During discharge, only similar to 60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMQ(2)-like domains are fully refilled (via reversible Ni4+ reduction into Ni2+), while the ordered Li2MnO3-like domains experience a Significant size decrease after the first cycle of charge/discharge. The originality of the present approach consists of analyzing NMR data with a new model that includes at its heart local electroneutrality constraints. This model allowed us to shed light on the processes Occurring in the Li-rich Mn/Ni layered oxide compound during the first electrochemical cycle on the microscopic level.