The process parametric window for diamond deposition using the chemical vapor deposition at low pressures is quite limited where addition of oxygen in the gas phase broadens this window. The lower boundary of the lens-shaped domain in CHO ternary diagram concurs with the H2CO tie-line (C/(C + O) = 0.5). In this work, we present the set of experiments where the ratio of C/(C + O) was kept at a constant value 0.385. The effect of hydrogen concentration (ratio O/(O + H) varied from 0.047 to 0.364) on plasma characteristics and deposited NCD films were investigated. Raman spectroscopy confirmed the diamond character of all deposited coatings while scanning electron microscopy showed transformation from not closed to continuous film and further decrease of grain size and finally growth of diamond nanowires while decreasing hydrogen concentration in a gas mixture.