  We discuss a method to obtain an effective oxygen target within a low-density
detector allowing an accurate characterization of the various event topologies
in $\nu (\bar \nu)$-oxygen interactions. Results can be of interest for
long-baseline neutrino oscillation experiments utilizing water targets. In
particular, the combination of both oxygen and hydrogen targets within the same
detector can provide in-situ measurements of nuclear effects and of the
(anti)neutrino flux, which are the leading sources of systematic uncertainties
in long-baseline oscillation analyses. These measurements can also provide
useful information about the nuclear modifications of bound nucleons, as well
as about the isospin symmetry in nucleons and nuclei.
