  Spectra obtained with the Hubble Space Telescope GHRS are combined with
high-resolution optical spectra and UV spectra from Copernicus to study the
abundances and physical conditions in the diffuse interstellar clouds seen
along the line of sight to the star 23 Ori. Strong low-velocity (SLV)
absorption, due to cool, moderately dense neutral gas and representing about
92% of the total N(H I), is seen for various neutral and singly ionized species
at +20 km/s < v < +26 km/s. Weaker low-velocity absorption, probably largely
due to warmer neutral gas, is seen primarily for various singly ionized species
at 0 km/s < v < +30 km/s. Absorption from a number of singly and doubly ionized
species, perhaps due to a radiative shock, is seen at -108 km/s < v < -83 km/s.
Weak absorption components from ionized gas are seen at intermediate velocities
(-43 km/s < v < -4 km/s). The large range in n_e (from 0.04 cm^{-3} to 0.95
cm^{-3}) derived independently from nine pairs of neutral and singly ionized
species in the SLV gas suggests that additional processes besides simple
photoionization and radiative recombination affect the ionization balance. The
adopted average SLV electron density, n_e = 0.15+/-0.05 cm^{-3}, implies a
relatively large n_e/n_H ~ 0.01, and thus some ionization of hydrogen in these
predominantly neutral components. Comparisons of the SLV depletions and n_H
with those found for the strong ``component B'' (v ~ -14 km/s) blend toward
zeta Oph hint at a possible relationship between depletion and local density
for relatively cold interstellar clouds. Calcium appears to be more severely
depleted in warm, low density gas than has generally been assumed. An appendix
summarizes the most reliable oscillator strengths currently available for a
number of the interstellar absorption lines analyzed in this work.
