  We use new Space Telescope Imaging Spectrograph (STIS) and archival Goddard
High Resolution Spectrograph (GHRS) observations to study interstellar B II
1362 and O I 1355 absorption along seven sightlines. Our new column density
measurements, combined with measurements of four sightlines from the
literature, allow us to study the relative B/O abundances over a wide range of
interstellar environments. We measure sightline-integrated relative gas-phase
abundances in the range [B/O] = -1.00 to -0.17, and our data show the B/O
abundances are anticorrelated with average sightline densities over the range
log <n_H> ~ -1.3 to +0.7. Detailed comparisons of the B II and O I line shapes
show that the B/O ratio is significantly higher in warm interstellar clouds
than in cool clouds. These results are consistent with the incorporation of
boron into dust grains in the diffuse ISM. Since boron is likely incorporated
into grains, we derive a lower limit to the present-day total (gas+dust)
interstellar boron abundance of B/H > (2.5+/-0.9)x10^-10. The effects of dust
depletion and ionization differences from element to element will make it very
difficult to reliably determine 11B/10B along most interstellar sightlines.
