A comparison of two OFD calculation procedures has been presented. The first procedure was based on a multiple registration of the voltammetric curve (5 replications) after one electrodes installation in the soil. The other procedure was based on the registration of the first voltammetric curve after each electrode reinstallation. Both accuracy parameters expressed as a variation coefficient were compared. A more accurate procedure was chosen. The procedure of OFD calculation in the unsaturated soil based on one registration of the voltammetric curve after each reinstallation (installation in another site in the soil) allows for the obtaining of higher precision than the procedure based on the multiple repetition of the voltammetric curve registration after one electrodes installation. An additional advantage of this procedure is the reduction of the measuring time, and in consequence, a lower probability of environmental changes during measurement due to oxygen absorption by the soil micro-organisms.