Annealing effect of high-purity granular ZnO up to 286 A degrees C has been studied by in situ Doppler broadening of electron-positron annihilated gamma-ray line shape measurement. Increase of S-parameter has been observed during annealing at 182 and 286 A degrees C which saturates in a time scale of similar to 90 min. The increase of S-parameter during 182 A degrees C annealing is related to the removal of carbon and loosely bound hydroxyl groups from ZnO, thereby increasing open-volume defects. During 286 A degrees C annealing, such open volumes agglomerate and migrate to grain surface regions causing further increase of open volumes at the positron annihilation site. Ex situ photoluminescence measurement has been carried out with samples annealed at 182 and 286 A degrees C. Room-temperature PL results are consistent with positron annihilation spectroscopic findings. Additionally, 10 K PL spectrum shows large increase of 3.311 eV emission in 286 A degrees C-annealed sample. This particular emission is related to typical crystal defects in ZnO which is a matter of discussion till date. The present study provides understanding on the interplay of defects in relatively low-temperature-annealed ZnO and is important from theoretical perspective as well as for improving the performance of ZnO as photocatalytic and gas-sensing agent.