Spectroscopic observations have been taken for 8 type-D symbiotic systems in the spectral regions lambda 3100 - 4100 and lambda 3700 - 5700. We show that the O III permitted lines formed by the O1 and O3 Bowen processes are surely present in these objects. In this paper the data have been used to calculate ratios of lines arising from the same upper level. The results indicate that the intermediate-coupling radiative rates calculated by Froese Fischer (1994) are the most accurate for O III Bowen fluorescence lines. The efficiencies of the Bowen mechanism for both processes are derived. In spite of the relatively small size of our sample, we find that the efficiencies increase with the fractional abundance of ionized helium and electronic density, but decrease with electron temperature, confirming previous results from studies of planetary nebulae.