Distinctive properties of Janus monolayer have drawn much interest in biotechnology applications. For this purpose, it has explored theoretically all sensing possibilities of nucleobases molecules (DNA/RNA) by Janus MoOS monolayer on both oxygen and sulfur terminations by means of rigorous first–principles calculation. Indeed, differences in interaction energy between nucleobases indicate that a monolayer can be used for DNA sequencing. Exothermic interaction energy range for DNA/RNA molecules with both oxygen and sulfur sides of the Janus MoOS surfaces have been found to range between (0.61–0.91 eV), and (0.63–0.88 eV), respectively, and the binding distances indicate that these molecules bind to both facets by physisorption. The exchange of weak electronic charges between the MoOS monolayer and the nucleobases molecules has been studied by means of Hirshfeld-I charge analysis. It has been observed that the introduction of DNA/RNA nucleobases molecules alters the electronic properties of both oxygen and sulfur atomic layers of the Janus MoOS complex systems as determined by plotting the 3D Kohn–Sham frontier orbitals. A good correlation has been found between the interaction energy, van der Waals energy, Hirshfeld-I, and d–band center as a function of the nucleobase’s affinity, and the interaction energy, suggesting adsorption dominated by van der Waals interactions driven by molybdenum d–orbital. Moreover, the lowering in the adsorption energy leads to an active interaction of the DNA/RNA with the surfaces, accordingly its conduct to shorter the recovery time. The selectivity of the biosensor modulation device has illustrated a significant sensitivity for the nucleobases on both the oxygen and sulfur layer sides of the MoOS monolayer. This finding reveals that apart from graphene, dichalcogenides–Janus transition metal may also be adequate for identifying DNA/RNA bases in applied biotechnology.