Nanocrystalline diamond (NCD) thin films are ideal to realize platform for bio-chemical sensors. They interface in a very specific way with the environment and the surface termination controls their electrochemical behavior. Generally the as-deposited NCD diamond surface is hydrogen terminated and several techniques are reported to obtain covalent bonding of biological matter. Among others, amino-functional groups are very versatile linkers to biomolecules. Grafting of amino groups to the diamond surface includes: chlorination and exposure to ammonia gas under thermal conditions, radio-frequency plasma or a one step method consisting of UV irradiation in a pure ammonia atmosphere. This last technique is very promising, combining surface functionalization with the realization of SGFETs (Solution Gate FET), and thus being compatible with semiconductor technologies. Experimental data from literature show that ammonia functional groups are always accompanied by the presence of oxygen which degrades the NCD conductivity. In this work we report on XPS in situ study to understand the role of oxygen in the one step amination process. Five NCD samples with different surface terminations (hydrogenated, plasma oxidized, chemically oxidized and UV oxidized) were irradiated with UV photons in different atmospheres (pure NH3 or NH3 + O2). XPS analysis shows that the presence of oxygen, both on the surface and in the gas mixture, strongly increases the efficiency of the UV surface amination.Research Highlights► NCD thin films were growth on silicon substrate using HFCVD process. ► Three NCD surfaces were oxidized using a plasma process, piranha solution and UV light in pure oxygen atmosphere. ► Two different gas atmosphere were considered to induce amino groups on diamond sample surface: ammonia or oxygen plus ammonia atmosphere. ► The role of oxygen in the one step amination process was investigated. ► Oxygen is essential to obtain NCD surface amination.