The OI puzzle
The 63 µm [OI] line is the main cooling line in the warm neutral medium (Wolfire, McKee, Hollenbach & Tielens 2003, ApJ 587, 278) and in photon-dominated regions with high density and UV field (Röllig, Ossenkopf, Jeyakumar et al. 2006, A&A 451, 917). Its understanding is thus essential to solve the energy balance of the interstellar medium.
ISO data
ISO observations have shown very small intensity ratios between the 63 and the 145 µm [OI] lines which cannot be explained by any single-component model for the emitting region (Lisea, Justtanont, Tielens 2006, A&A 446, 561). This currently prevents the reliable determination of both oxygen temperatures and abundances leaving the energy balance problem open.
Millimetron observations
The discrepancy in the line ratio could be explained by optically thick emission from very dense, high column density PDRs, by steep temperature gradients leading to self-absorption or by foreground absorption. The first would manifest itself in flat-top line profiles, the second in symmetric profiles with self-absorption dips and the last possibility would result in irregular profiles with multiple components.
None of the existing missions was able to resolve the 63 µm [OI] line profiles, so that the possible solutions remain speculations at the moment. Only Millimetron will be able to resolve the line profiles, give an explanation for the line ratios and determine the actual oxygen abundances. This will finally solve the energy balance problem for the warm neutral interstellar medium.