<p>Electron spin resonance (ESR) spectra of manganite perovskite LaMnO_3+δ are discussed as a function of oxygen excess, <i>δ</i>. For this reason, samples of LaMnO_3+δ with a wide range of <i>δ</i> values (0.05&#xa0;≤&#xa0;<i>δ</i>&#xa0;≤&#xa0;0.24), have been prepared via a co-precipitation process followed by calcination in air at different temperatures. The values of <i>δ</i> were determined by iodometric titration. X-ray diffraction analysis revealed that the structure of LaMnO_3+δ is susceptible to the oxygen content, and it changes from orthorhombic for <i>δ</i>&#xa0;=&#xa0;0.09 to rhombohedral for <i>δ</i>&#xa0;≥&#xa0;0.13. The significant change in the unit-cell parameter and the volume per formula unit (<i>V</i>_<i>f</i>), which accompany this transformation, cannot be explained by steric or electronic effects. Over-stoichiometry <i>δ</i> induces a mixed valence state (Mn³⁺, Mn⁴⁺) and a decrease in the Jahn–Teller effect: with more open Mn-O-Mn angles, these would favor the double-exchange (DE) interaction. However, the increase in the proportion of Mn vacancies disrupts the connection paths and makes the DE coupling weaker. The influence of the observed defective structures on the ESR measurements is discussed. The ESR spectra recorded at room temperature are symmetrical lines in the Lorentzian shape with a resonance field corresponding to values of <i>g&#xa0;=&#xa0;</i>1.996, and are characteristic of a paramagnetic regime. The changes in the percentage of Mn³⁺ and Mn⁴⁺ vacancies at the B-sites and the Mn-O-Mn bond angle shows that the ESR response is managed by bottlenecked spin relaxation taking place from the exchange-coupled Mn⁴⁺ via Mn³⁺ to the lattice. For LaMnO_3.24, the increase in ΔI_pp with the narrowest ESR linewidth, ΔH_pp , was interpreted as the formation of a charge order (Mn³⁺/Mn⁴⁺) involving local DE to form a Zener polaron. The charge ordering is observed for half-doped manganites of La_0.5A’_0.5MnO₃ where A’ is a divalent cation.</p>