  Far infrared fine structure line data from the ISO archive have been
extracted for several hundred YSOs and their outflows, including molecular (CO)
outflows, optical jets and Herbig-Haro (HH) objects. Given the importance of
these lines to astrophysics, their excitation and transfer ought to be
investigated in detail and, at this stage, the reliability of the diagnostic
power of the fine structure transitions of O I and C II has been examined.
Several issues, such as the extremely small intensity ratios of the oxygen 63
micron to 145 micron lines, are still awaiting an explanation. It is
demonstrated that, in interstellar cloud conditions, the 145 micron line is
prone to masing, but that this effect is likely an insufficient cause of the
line ratio anomaly observed from cold dark clouds. Very optically thick
emission could in principle also account for this, but would need similar,
prohibitively high column densities and must therefore be abondoned as a viable
explanation. One is left with [O I] 63 micron self absorption by cold and
tenuous foreground gas, as has been advocated for distant luminous sources.
Recent observations with the submillimeter observatory Odin support this
scenario also in the case of nearby dark molecular clouds. On the basis of this
large statistical material we are led to conclude that in star forming regions,
the [O I] and [C II] lines generally have only limited diagnostic value.
