The possibility of using a fast neutron beam with the associated alpha particle (AAP) technique in conjunction with a specially designed probe for carbon-to-oxygen (C/O) nuclear well logging was investigated. Measurements were performed to show the influence of borehole fluid, iron casing, and the cement around the casing in the C/O analysis. Tests were conducted by using fast neutron activation analysis (FNAA), as the conventional approach for C/O measurements in well logging and by using AAP. Analyzed samples were made from quartz sand and graphite powder mixed in different C/O ratios characteristic for the oil-containing formations. Diesel fuel was used as a simulant of the borehole fluid. Monte Carlo (MC) simulations were conducted and the results were compared with the experimental data. In both tests, the linear relation between the measured and real C/O ratios was obtained, but when the FNAA was used, the linear relation was significantly influenced mainly by the presence of the borehole fluid. Contrary to FNAA, the AAP technique resulted in a significant reduction of the background signal coming from the borehole fluid.