Complex oxides exhibit many fascinating physical phenomena that make them perfect candidates for fundamental and applied research. Their electronic and photonic properties strongly depend on their native defects. We can fully characterize such defects in terms of their level in the band gap and their type and structure by combining stimulated luminescence techniques with positron annihilation spectroscopy (PAS). Positron annihilation is unique probe to identify atomic size defects with 10-7 sensitivity. In this talk I will discuss the basics of PAS and its significant role in advancing semiconductors and electronic and photonic materials research. Then I will share some of our interesting results on applying PAS to understand the optical and transport properties of a few multi-functional complex oxides such SrTiO3 (STO). The focus will be on our recent discovery: of 1) Unusual luminescence phenomena in STO and 2) large persistent photoconductivity in its bulk with negligible decay, and the role of PAS in revealing their origins. Lastly I will discuss the implications on our understanding of some novel phenomena such as the formation of 2-dimensional electron gas (2DEG) and Metal to Semiconductor Transition at the interface of STO based heterostructures.