Using the GREAT instrument aboard the airborne SOFIA observatory, we have imaged [O I] 63.18 micron line emission from a bright photodissociation region (PDR) associated with an ionized “bubble” located in the Nessie Nebula, a filamentary infrared dark cloud. The [O I] line profiles reveal self-absorption by cold, foreground gas against line emission from the warmer gas associated with the photodissociation region, as well as absorption against the brightest far-infrared continuum source associated with a high-mass protostar. A standard PDR analysis yields implausibly low densities or implausibly high UV field strengths if the observed [O I] fluxes are used. However, if self-absorption diminishes the actual [O I] flux from the PDR by factors of ~5, the PDR models yield sensible values. Thus, large optical depths in foreground, cooler (predominantly molecular) gas can hinder the utility of the [O I] line as an accurate diagnostic of PDRs.