Display omitted •A new organic–organic salt and its deuterium analogue were characterized.•The DSC measurements exhibit that AMNPO undergoes a reversible phase transition at ∼240K.•The AMNPO crystals are built of ions which are connected via NH⋯O and OH⋯O interactions.•Combined X-ray, IR, RS and DFT methods applied to hydrogen bonds.•The temperature dependence of hydrogen bonds band wavenumbers was analyzed. A new organic–organic salt, 2-amino-4-methyl-3-nitropyridinium hydrogen oxalate (AMNPO), and its deuterium analogue have been synthesized and characterized by means of FT-IR, FT-Raman, DSC and single crystal X-ray studies. The DSC measurements and temperature dependence of the IR and Raman spectra in the range 4–295K show that it undergoes a reversible phase transition at ∼240K. At room temperature it crystallizes in noncentrosymmetric space group P21. The unit-cell is built of the 2-amino-4-methyl-3-nitropyridinium cations and oxalate monoanions which are connected via the NH⋯O and OH⋯O hydrogen bonds. The geometrical and hydrogen bond parameters are similar for non-deuterated (at 120 and 293K) and deuterated compounds (at 90K). The phase transition is probably a consequence of order–disorder transition inside of hydrogen network. The 6-311G(2d,2p) basis set with B3LYP functional have been used to discuss the structure and vibrational spectra of the studied compound.