In this article, we combined two complementary structure/dynamics probes, i.e., total-scattering/reverse Monte Carlo (RMC) modeling and classical molecular dynamics (MD), in order to understand local lithium structure and dynamics in a model disordered garnet oxide Li5La3Ta2O12. By examining the configurations from RMC and trajectories from MD, we individually and statistically analyzed the lithium distribution and dynamics within tetrahedral (Td) cages, octahedral (Oh) cages, and triangular bottlenecks. We found that lithium atoms within either Td or Oh cages prefer to stay at the off-center positions and close to one of the triangular bottlenecks. This is likely caused by the uneven Li-Li interaction in the form of lithium clusters, and such geometrical frustration leads to the local structure instability and fast ionic conduction. Both RMC and MD studies support that the lithium conduction path goes through the triangular bottleneck in a 3D continuous network of Td/Oh cages, without a direct Oh to Oh jump. However, the conduction mechanism should not be generalized, as it is greatly influenced by the local environment or temperature. Broadly speaking, lithium atoms hop through the bottleneck from an edge-passing mechanism at low temperatures to a center-passing mechanism at higher temperatures.