Conformations and solvation of oligomeric and polymeric plant tannins in mixed water-ethanol solvents were studied by light scattering and small angle X-rays scattering (SAXS). We find that structures are not simple linear chains but are ramified with about 6% of branching monomers. Ab initio reconstruction reveals that monomers within a single branch are closely bound in pairs. Structure is rather rigid, with the Kuhn length b=13 nm, corresponding to 30 linearly bound monomers. A non-montonous dependence on ethanol content was found in the SAXS signal from solvation layer, indicating amphipathic character of molecules. Resulting best solvent composition is a decreasing function of polymerization degree N, in agreement with increasing water solubility of tannins with N. Polymers longer than b present a power-law behavior 1/Q^d in the SAXS profile at high momentum transfer Q. Dimension d decreases monotonously with ethanol content, from 2.4 in water to 1.9 in ethanol, because the molecules are more compact in water than in ethanol due to attractive water-induced intra-molecular interactions. Same tannins were then oxidized in controlled conditions similar to real biological or food systems. We show that oxidation does not produce any inter-molecular condensation, but it generates supplementary intra-molecular links. Some of oxidation products are insoluble in water rich solvent. For that reason, we identify these species as T1 tannins in the notation of [Zanchi et al., Langmuir 23, 9949 (2007)]. Larger fraction remains soluble in all water-ethanol compositions. Conformations of oxidized polymeric tannins, in despite of their higher rigidity, remain sensible to solvent composition.