Despite having been used for ages to protect wood against the influence of outdoor elements, the chemistry of vegetable oils within wood is poorly known. We propose a method based on solid-state magic-angle sample spinning NMR to in situ characterize oil oxidation as well as its immobilization. To eliminate signal coming from wood molecules but to keep signal from the oil, direct 13C excitation is performed with low-power 1H decoupling during signal acquisition. To suppress the effect of anisotropic spin-interactions and magnetic field inhomogeneity, the sample is spun at the magic-angle. Mono- and polyunsaturated fatty acid derivatives show a difference in their oxidation process: the monounsaturated methyl oleate reacts with wood components and becomes immobilized while the polyunsaturated methyl linoleate becomes oxidized and form oligomers but does not seem to bind to wood. Linola® oil behaves as would be expected on the basis of its composition by monounsaturated and polyunsaturated chains. This method can be generalized to all coating treatments to characterize chemical pathways and reactions. A better understanding of coating effects on wood is a crucial step to design more efficient protective mixtures.