Ultrathin Bi₂O₂Se nanostructures possess ubiquitous structural and optical properties suitable for future optoelectronics. Herein, we have studied the unique structural and optoelectronic functionalities of ultrathin Bi₂O₂Se nanocrystals, synthesized via a scalable chemical reaction process with high reproducibility, and their modulation through vacuum annealing. The as-prepared nanosheets (NS) with monolayer-to-few-layer thickness possess different kinds of defects (e.g., Bi-antisites at Se vacancies, Se vacancies, interstitial O, O antisites at Se vacancies) due to their free-standing nature with Se-terminated charged surfaces. Our study reveals that vacuum annealing improves the crystal quality through defect passivation, basically through inter-layer self-assembly via electrostatic interactions. Interestingly, we observed self-powered negative persistent photoconductivity (PPC) in the as-grown defect-containing Bi₂O₂Se NS and positive photoconductivity (PC) in annealed Bi₂O₂Se NS containing fewer defects. The mechanistic origin of such a conversion from negative PPC to positive PC is discussed in detail, where the energy levels created by the defects serve as photo-gates. In addition, photoresponsivity (vis–NIR) measurements show a photo-gating effect created by the defects that play a key role in modulating the wavelength-dependent photoconductivity. The peak responsivity of the device is shown to be 0.4 A W⁻¹ at 808 nm. This work highlights the possible origin of the crystallinity improvement of non-van der Waals Bi₂O₂Se nanosheets through vacuum annealing and their intriguing photoresponse properties in the visible-to-near-infrared range, which are of fundamental importance and practical application in cutting-edge optoelectronics.