We propose to estimate the Organic Carbon (OC) stocks for the O soil layers, at the national scale. First, the determinants of the OC content variability in soil O-horizons were examined, then the OC content was mapped at the national scale and this map was used to estimate the OC stocks for French forest soils. Three soil datasets were used. Two were national datasets provided by national soil survey programs for soils knowledge and inventory and soil quality survey (IGCS and RMQS). One was provided by the Level I of the European program BioSoil. Various covariates were used to identify the determinants of OC stocks in O-horizons: the soil descriptions provided by the pre-mentioned datasets, with collection of biological, chemical and physical properties of O-horizons and soils, plus a description of surrounding landscapes and climate properties. For OC content mapping in O-horizons, spatially exhaustive covariates were used, such as a digital elevation model (SRTM DEM, 90-m grid) and its derived attributes, the French soils map (1:1,000,000), climatic and land use data. Predictive models of OC stocks in the O-horizons were created using Generalized Boosted Regression Models. Estimation of uncertainties has been afterwards modeled using fuzzy k-means clustering. The final OC stocks in O soil layers were estimated to 15 TgC.