Domain engineering via oxygen vacancy, VO loading achieved by A/B modification as well as quenching treatment, was utilized for Ba0.8Sr0.2TiO3 (0.8 BSTs) in an attempt to enhance the microwave tunable characteristics. For similar grain sizes, the domain sizes were notably reduced for all nonstoichiometric BSTs, indicating that the loaded VO.. (as a consequence of Ti defects, VTi) played a role in the nuclei for new domain walls. The tunability T at 100 MHz under a direct current field of 30 V/20 m increased steadily as the domain size (d.s.) declined for all BSTs, regardless of the A/B ratio, due to the d.s. effect. The tunable characteristics in nonstoichiometric BSTs having a similar d.s. of 190 nm were then compared. The tunability and tan decreased for A/B = 1.002 (0.2 mol% Ti defects). The introduced VO.. formed pinning centers that restricted domain wall motion, leading directly to lower tunability and smaller dielectric loss. However, VO..-overloaded samples (i.e., A/B 1.005) exhibited increased values for tan due to VO.. conduction in the domains. The quench treatment of 0.8 BST (with A/B = 1.002) samples resulted in a d.s. reduction from 191 to 170 nm. These quenched specimens showed greater tunability, T-total, originating from the strengthened dipole contribution, T-dipole, as a consequence of the d.s. effect. The tan of the quenched specimens was essentially unchanged, indicating a homogenous VO.. distribution via the quench, effectively reducing the mobile VO.. (which contributes to electrical conduction) in the domains. Consequently, the achieved figure of merit via domain engineering was 2.25 at 100 MHz for the quenched BST with A/B = 1.002, which was 1.54 times larger than that of unmodified BST.