We report on electrical transport and quantum confinement in thermally annealed Si/SiO_x multiple quantum well (QW) stacks. Results are correlated with the morphology of the stacks. High temperature annealing of Si/SiO_x stacks leads to precipitation of excess Si from the SiO_x layers, which enhances the degree of crystallization and increases the grain sizes in the Si QWs compared to the conventional Si/SiO₂ system. Moreover, the excess Si forms highly conductive pathways between adjacent Si QWs that are separated by ultrathin silicon oxide barriers. This results in an increase of conductivity by up to 10 orders of magnitude compared to the tunneling dominated transport in Si/SiO₂ stacks. The stacks exhibit a distinct quantum confinement as confirmed by photoluminescence measurements.