Negative and low thermal expansion materials found applications in various fields. The compound Ag₂O shows negative thermal expansion (NTE) behavior up to its decomposition temperature of about 500 K. We have carried out neutron inelastic scattering measurements on a polycrystalline sample of Ag₂O. The phonon spectra have been analyzed using ab-initio calculations. The calculated pressure dependence of the phonon modes are used to calculate the thermal expansion behavior of Ag₂O. The calculations show that the low-energy rotational modes of Ag₄O tetrahedra are responsible for the large negative thermal expansion in Ag₂O.