The possibility of using the fast neutron beam with the associated alpha particle technique (APT) by the specially designed tool for Carbon-to-Oxygen (C/O) nuclear well logging was investigated. Measurements were done to show the influence of borehole fluid, iron casing, and the cement around the casing on the C/O analysis. Tests were conducted by using fast neutrons, which is the conventional approach for C/O analysis in well logging, and by using fast neutrons with APT. Analysed samples were made from quartz sand and graphite powder with the different C/O ratio characteristics for the oil-containing formation around the borehole. Diesel fuel was used as a simulant of the borehole fluid. Monte Carlo (MC) simulations were conducted and the MC results were compared to the experimental data. In both tests, the linear relation between the measured and real C/O ratios was obtained, but in the case in which the APT technique was not used, the linear relation was significantly influenced by the presence of the borehole fluid. The use of the APT technique resulted in a significant reduction of a background signal (coming mainly from the borehole fluid) in comparison to the conventional technique.