We present homogeneous carbon and oxygen abundances for a sample of 757 FGK stars in the HARPS-GTO sample, from which 101 stars host Jupiter-mass companions and 22 hosts Neptunian-mass companions. We derive C/O ratios using oxygen abundances from the 6300A [OI] forbidden line (low excitation potential) and with the 6158A line (high excitation potential as the CI optical lines used to determine carbon abundances). We find a tentative evidence that stars with low-mass planets at lower metallicities have higher [C/Fe] ratios than stars without planets at the same metallicity, in the same way as previously found for alpha elements and Zn. The elemental C/O ratios for the vast majority of our stars are below 0.8 when using the oxygen line at 6158Å but the forbidden oxygen line at 6300Å provides systematically higher C/O values (going above 1.2 in few cases) which also show a dependence on Teff. Moreover, by using different atmosphere models the C/O ratios can have a non negligible difference for cool stars. Therefore, C/O ratios should be scaled to a common solar reference in order to correctly evaluate its behaviour. We find no significant differences in the distribution of C/O ratios for the different populations of planet hosts except when comparing the stars without detected planets with the stars hosting Jupiter type planets. However, we note that this difference might be caused by the different metallicity distributions of both populations.