Substitution of La for Ba in LaBaGaO melilite yields a new interstitial-oxide-ion conducting LaBaGaO solid solution, which only extends up to x = 0.35, giving a maximum interstitial oxygen content allowed in LaBaGaO as about half of those allowed in La(Sr/Ca)GaO. LaBaGaO ceramic displays bulk conductivity ∼1.9 × 10 S/cm at 600 °C, which is lower than those of La(Sr/Ca)GaO, showing the reduced mobility for the oxygen interstitials in LaBaGaO than in La(Sr/Ca)GaO. Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in LaBaGaO are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the c-axis and stabilized via incorporating into the bonding environment of a three-linked GaO among the five GaO tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGaO has the largest formation energy for oxygen interstitial defects among LaMGaO (M = Ba, Sr, Ca), consistent with the large Ba cations favoring interstitial oxygen defects in melilite less than the small cations Sr and Ca. The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in LaMGaO (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba-containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels.