Oxygen is a ubiquitous element, playing an essential role in most scientific and technological
disciplines, and is often incorporated within a structurally disordered material where examples
include molten silicates in planetary science, glasses used for lasers and optical
communication, and water in biological processes. Establishing the structure of a liquid or
glassy oxide and thereby its relation to the functional properties of a material is not, however, a
trivial task owing to the complexity associated with atomic disorder. Here we approach this
challenge by measuring the bound coherent neutron scattering lengths of the oxygen isotopes
with the sensitive technique of neutron interferometry. We find that there is a small but finite
contrast of 0.204(6) fm between the scattering lengths of the isotope 18O and oxygen of natural
isotopic abundance natO, contrary to tables of recommended values. This has enabled us to
investigate the structure of both light and heavy water by exploiting, for the first time, the method
of oxygen isotope substitution in neutron diffraction, thus circumventing many of the
significant problems associated with more traditional methods in which hydrogen is substituted
by deuterium. We find a difference of ~0.5% between the O-H and O-D intra-molecular
bond distances which is much smaller than recent estimates based on diffraction data and is
found to be in excellent agreement with path integral molecular dynamics simulations made with
a flexible polarisable water model. Our results demonstrate the potential for using oxygen
isotope substitution as a powerful and effective site specific probe in a plethora of materials, of
pertinence as instrumentation at next generation neutron sources comes online
