In this study, we present six structural models for amorphous Ga2O3 (a-Ga2O3) with different mass densities from ab initio molecular dynamics simulations. The detailed local structural order, non-perfect coordination number, electronic and vibrational properties of a-Ga2O3 as well as the energetics and stability of O vacancy with various charge states have been systemically investigated. As the density increases, the first peak in the radial distribution function is weakened and the range of vibrational frequencies is narrowed, while the average coordination number and band gap gradually increase. Density also plays an important role in oxygen vacancy formation in a-Ga2O3. In the low-density a-Ga2O3, oxygen vacancy is more stable and the average formation energy of oxygen vacancy (V-O) with neutral state is 3.32-3.70 eV lower than that in crystalline beta-Ga2O3 (4.20 eV). In the high-density a-Ga2O3, V-O in the 2 + charge state may form spontaneously in the case of lower Fermi energy and the thermodynamic transition energy for V-O was found to shift to higher Fermi energy. Hence, the electron trap levels observed in experiment might be correlated with local structural distortion and non-perfect coordination number. Moreover, highly localized valence tail has been observed in all the six a-Ga2O3 systems. Thus, it would be hard to shift the Fermi level to the valence band, leading to rarely observed p-type conductivity in amorphous Ga2O3. These results not only establish a fundamental picture on the origin of a wide range of defect levels in Ga2O3 thin films deposited under different conditions, but also provide useful evidence to identify them with oxygen vacancy states.