The bi-oleothermal© process (combination of oil and heat treatment) is a well mastered alternative method for wood protection. However, the fire behavior and resistance to decay of bi-oleothermally treated wood are not good enough to ensure performance which meets the service standards for outdoor applications such as cladding or decking expected by the market. The aim of the present research project has been to improve this performance by optimizing the linseed oil formulations used at the impregnation stage. Different formulations combining linseed oil, fungicides, insecticides and/or fire retardants were tested under laboratory conditions in order to assess the resistance of oil-treated wood to molds, decay fungi, longhorn beetles, subterranean termites and fire. The results showed that the investigated biological organisms exhibit different levels of susceptibility to oil based formulations. Additionally, the collected data suggested that some inhibition processes might occur between the oil and the active ingredients, lowering the biocide effect of the final formulation. Subsequently, chemical analyses were performed in order to identify the active ingredients both in the oil formulations used for the second bath and inside the treated wood. The measured concentrations of active ingredients were then compared to the expected target values to determine the possible fate of the biocides in the oil formulations (degradation, migration into wood, interaction between the different components).