Advanced modelling techniques have been used to investigate the defect properties of the LaGaO₃ ionic conductor on the atomic-scale. The present study addresses a variety of topics, but focuses on the site-selectivity of a wide range of dopants, the influence of A-site deficiency, the nature of dopant–vacancy association, and the energetics of oxidation in the transition-metal doped oxide. Ion size trends are found for both divalent and trivalent dopant incorporation at La and Ga sites. Certain trivalent dopants may show “amphoteric” behaviour in which the dopant can occupy either site, and is found to be influenced by A-site deficiency. The M⁴⁺ (Ti, Sn, Zr, Hf) and M⁵⁺ (Nb, Ta) dopants are predicted to substitute for Ga with La vacancy compensation. Dopant–vacancy clusters for a wide range of dopant ions have been examined. A minimum in the binding energy occurs for Sr²⁺ on La³⁺, which would be beneficial to oxygen ion conductivity. Dopant–vacancy association for isovalent (M³⁺) substitution indicates elastic strain effects. We have considered complex clusters () of x up to 12 within 2D and 3D structures, which may be related to possible “nano-domain” formation at higher dopant regimes. Favourable oxidation on energetic grounds is found for Ni²⁺ and Cu²⁺ doping, suggesting p-type conductivity at increasing oxygen partial pressures.