Room-temperature measurements of the Buckingham effect (electric-field-gradient-induced birefringence, EFGIB) for gaseous oxygen are presented. The traceless electric quadrupole moment of the oxygen molecule has been deduced from these data, with the assumption that the temperature-independent hyperpolarizability contribution to the EFGIB is negligibly small. The value obtained is Theta = (-1.033 +/- 0.027) x 10(-40) Cm-2. This value is compared with the best available ab initio quantum computational values in the literature. (C) 2014 Elsevier BM. All rights reserved.