  We present measurements of the oxygen abundance of the Milky Way's ISM by
observing the K-shell X-ray photoionization edge towards galaxy clusters. This
effect is most easily observed towards objects with galactic columns (n_H) of a
few times 1e21 cm^-2. We measure X-ray column densities towards 11 clusters and
find that at high galactic columns above approximately 1e21 cm^-2 the X-ray
columns are generally 1.5--3.0 times greater than the 21 cm H II columns,
indicating that molecular clouds become an important contributor to n_H at
higher columns. We find the average ISM oxygen abundance to be (O/H) = (4.85
+/- 0.06) x 10^-4, or 0.99 solar when using the most recent solar photospheric
values. Since X-ray observations are sensitive to the total amount of oxygen
present (gas + dust), these results indicate a high gas to dust ratio. Also,
the oxygen abundances along lines of sight through high galactic columns (n_H)
are the same as abundances through low columns, suggesting that the composition
of denser clouds is similar to that of the more diffuse ISM.
