  We present analysis of OI 63 micron and CO $J$ = 5-4 and 8-7 multi-position
data in the W3A region and use it to develop a model for the extended
low-density foreground gas that produces absorption features in the OI and $J$
= 5-4 CO lines. We employ the extinction to the exciting stars of the
background HII region to constrain the total column density of the foreground
gas. We have used the Meudon PDR code to model the physical conditions and
chemistry in the region employing a two-component model with high density layer
near the HII region responsible for the fine structure line emission, and an
extended low density foreground layer. The best-fitting total proton density,
constrained largely by the CO lines, is $n$(H) = 250 cm$^{-3}$ in the
foreground gas, and 5$\times$10$^5$ cm$^{-3}$ in the material near the HII
region. The absorption is distributed over the region mapped in W3A, and is not
restricted to the foreground of either the embedded exciting stars of the HII
region or the protostar W3 IRS5. The low-density material associated with
regions of massive star formation, based on an earlier study by Goldsmith et
al. (2021), is quite common, and we now see that it is extended over a
significant portion of W3A. It thus should be included in modeling of fine
structure line emission, including interpreting low-velocity resolution
observations made with incoherent spectrometer systems, in order to use these
lines as accurate tracers of massive star formation.
