The interactions of prime interstitial and alloying elements in hexagonal Ti were investigated using a density functional theory calculations. The binding energies of oxygen with all substitution elements whose solubility limit in alpha-Ti is greater than 3 at% were calculated. The investigations performed reveal no attraction between Zn, Zr, Ag and O, and strong O-Sc and O-Sn binding. It was found that the O-X clustering mechanism is based on a direct and long-range O-X interaction, both controlled by valence structure and electronegativity of substitational elements. The single crystal and isotropic elastic constants together with Pugh's plasticity criterion were calculated for Ti with multiple point defects to evaluate their impact on mechanical properties. The results obtained reveal that a low concentration of O improves ductility in Ti + Sc solid solutions and increases the brittleness of Ti + Sn alloys. The diverse effect on ductility is due to different chemical bond types in the vicinity of O. The results show that the interstitial-substitational elements clustering effect may be used to optimize mechanical properties of alpha-Ti alloys. (C) 2015 Elsevier B.V. All rights reserved.