Abstract Recent systematic experimental studies, involving the benchmark Ti-6Al-4V alloy fabricated from powder, have established that there exists a critical level of oxygen around 0.33 mass% beyond which the tensile ductility of the alloy drops dramatically until reaching total brittleness. To understand the fundamental mechanisms behind this critical oxygen content, three-dimensional atom probe tomography (3D-APT), transmission electron microscopy (TEM) and other analytical means have been used to identify and characterise the fine microstructural changes induced by the increased oxygen content beyond the critical level. Three fine microstructural features have been identified in as-sintered Ti-6Al-4V when the interstitial oxygen content was increased from 0.25 mass% to 0.49 mass%. These include (i) the formation of fine acicular α precipitates in the β phase, (ii) the formation of α2-type (Ti3Al) nanometric clusters in the α matrix, and (iii) grain boundary α-β-α layered structures between the α grains. The impacts of these microstructural changes on the tensile ductility are discussed.