Spiky polycrystalline SnO2 balls assembled from pyramid-shaped SnO2 nanocrystals with surface oxygen vacancies were synthesized by a simple and facile in-situ molten salt oxidation method using SnCl2 as the tin source and graphite as the surface reductant. In this method, perfect SnO2 nanocrystals are first formed on the surface of graphite particles at 500 degrees C. By increasing the processing temperature to 700 degrees C, the formation of surface oxygen vacancy defects was confirmed, supported by various characterisation methods including Raman and XPS spectroscopy. A sharp increase in the bulk electrical conductivity of the samples was detected by exceeding the onset temperature corresponding to the formation of oxygen deficient SnO2 crystals, at which the conductivity of the sample significantly outperformed that of commercial SnO2 nanoparticles. The formation of oxygen vacancies on SnO2 crystals is thermodynamically studied, and suggested to occur by surface carbon reduction of the crystals at sufficiently high temperatures. The enhanced electrical conductivity of oxygen deficient SnO2 crystals can lead to a wider application of tin oxides in advanced electrical applications.