In order to elucidate oxygen nonstoichiometry, defect structure, and defect equilibrium relation in La_2-xSr_xNiO_4+δ (x = 0, 0.2, 0.4), defect chemical and statistical thermodynamic analyses were carried out on the experimentally evaluated oxygen content variation. The relationship between δ, P(O₂), and T were analyzed by a defect equilibrium model. Since La_2-xSr_xNiO_4+δ shows p-type metallic conduction at high temperatures, chemical potential of hole was re-defined from the integration of the Fermi-Dirac distribution function. The nonstoichiometric variation of oxygen content in La_2-xSr_xNiO_4+δ can be explained by the defect equilibrium model with the assumption of the itinerant electronic state, the rigid-band approximation and the regular solution approximation for interstitial oxygen formation. Partial molar entropy and enthalpy of oxygen were calculated from the nonstoichiometric data and Gibbs-Helmholtz equation. The relationship among defect structure, defect equilibrium, and thermodynamic quantities was elucidated by the statistical thermodynamic model using the results of defect chemical analysis. Proposed defect equilibrium and statistical thermodynamic models can explain the δ-T-P(O₂) relationship and the thermodynamic quantities very well, meaning these models can predict defect structure of La_2-xSr_xNiO_4+δ under a certain condition.