We present an analysis of high resolution HST Space Telescope Imaging Spectrograph ( STIS ) observations of O i \lambda 1356 and H i Lyman- \alpha absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f ( H _ { 2 } ) , over 2 orders of magnitude in { \langle } n _ { H } \rangle , and that extend up to 6.5 kpc in length .
Since the majority of these sight lines have also been observed by the Far-Ultraviolet Spectroscopic Explorer ( FUSE ) , we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard High-Resolution Spectrograph ( GHRS ) results .
Two distinct trends are identified in the 56 sight line sample : an apparent decrease in gas-phase oxygen abundance with increasing mean sight line density ( { \langle } n _ { H } \rangle ) and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc .
The first effect is a smooth transition between two depletion levels associated with large mean density intervals ; it is centered near { \langle } n _ { H } \rangle = 1.5 { cm } ^ { -3 } and is similar to trends evident in gas-phase abundances of other elements .
Paths less dense than the central value exhibit a mean O/H ratio of log _ { 10 } ( O/H ) = -3.41 \pm 0.01 ( or 390 \pm 10 ppm ) , which is consistent with averages determined for several long , low-density paths observed by STIS ( 1 ) and short low-density paths observed by FUSE ( 40 ) .
Sight lines of higher mean density exhibit an average O/H value of log _ { 10 } ( O/H ) = -3.55 \pm 0.02 ( 284 \pm 12 ppm ) .
The datapoints for low- { \langle } n _ { H } \rangle paths are scattered more widely than those for denser sight lines , due to O/H ratios for paths shorter than 800 pc that are generally about 0.10 dex lower than the values for longer ones .
Scenarios which would be consistent with these results include a recent infall of metal-poor gas onto the local Galactic disk and an interstellar environment toward Orion that is conducive to reducing the apparent gas-phase oxygen abundance .