In this paper, we prepare heavily electron doped $\mathrm{L}{\mathrm{a}}_{0.04}\mathrm{B}{\mathrm{a}}_{0.96}\mathrm{Sn}{\mathrm{O}}_{3}$ (LBSO) thin films on a $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO) substrate by the pulsed laser deposition method under different oxygen pressures, in which different levels of oxygen vacancy concentrations are present. Normally, oxygen vacancies are implicitly assumed as isolated point defects that dope the materials with electron carriers, whereas oxygen-deficient LBSO films exhibit an apparent metal-to-insulator transition combined with lower carrier concentrations and mobility with increased oxygen vacancies, which are concluded from the transport measurement data. Considering the almost constant density of threading dislocations and the higher formation energies for other intrinsic crystal defects, the observed transport trends are ascribed to the extra oxygen vacancies introduced. Therefore, strong electron localization originating from the interaction between the oxygen vacancy and La impurity is proposed for interpreting the behavior in oxygen-deficient LBSO films. Oxygen-deficient crystal structure models for LBSO have been optimized and the electronic structures are revealed by first-principles calculations based on the density functional theory. The partial density of states results indicate that strong electron localization comes from the deep strongly localized states in the forbidden gap, which are mainly composed of hybridized orbitals of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ with $\mathrm{Sn}\phantom{\rule{0.16em}{0ex}}5s5p$ and some amount of $\mathrm{Sn}\phantom{\rule{0.16em}{0ex}}4d$. Thus, the control of the oxygen defects and the related electronic states in barium stannate is essential for achieving optimal electrical performance and as potential applications in perovskite heterostructure devices.