Ruddlesden-Popper (RP) nickelates are a promising class of high-temperature superconductors, with superconducting transition temperatures exceeding the boiling point of liquid nitrogen. However, oxygen nonstoichiometry remains a persistent challenge that commonly presents in all RP nickelates. Understanding the formation of oxygen vacancies, their ordering patterns, and their impact on superconductivity is crucial, especially in the newly discovered $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$. In this study, the first-principles structural calculations reveal the formation of in-plane oxygen vacancy chains in $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$, a key structural feature observed under both ambient and pressurized conditions. These vacancies induce significant lattice distortion and generate residual electrons that hybridize with the Ni ${d_z}_2$ orbital, altering the sign of the hopping integral between the ${d_z}_2$ orbitals. Furthermore, the vacancies alter the $\mathrm{N}{\mathrm{i}}^{2+}\text{/}\mathrm{N}{\mathrm{i}}^{3+}$ ratio, lower the ${d_z}_2$ orbital energy, and decrease the ${d_z}_2$ orbital density of states at the Fermi level. Interestingly, at higher vacancy concentrations, the vacancy chains tend to align diagonally along the out-of-plane direction. The phase diagram of $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ exhibits a narrow stability range at ambient pressure, which expands under applied pressure, aligning with the high-oxygen-pressure conditions required for its synthesis. Importantly, these vacancy chains broaden the optical conductivity peak, which could serve as a marker for their detection. Our findings offer valuable insights into the distribution of oxygen vacancies, their role in modifying the electronic structure, and their influence on optical conductivity in $\mathrm{L}{\mathrm{a}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$.

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• Defects
• Structural properties
• Nickelates
• Superconductors
• First-principles calculations