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Observations of the four $^{2}\Pi_{3/2},~J = 3/2$~ground state transitions ofthe hydroxyl radical (OH) have emerged as an informative tracer of moleculargas in the Galactic ISM. We discuss an OH spectral feature known as the `flip',in which the satellite lines at 1612 and 1720\,MHz flip -- one from emission toabsorption and the other the reverse -- across a closely blended doublefeature. We highlight 30 examples of the flip from the literature, 27 of whichexhibit the same orientation with respect to velocity: the 1720\,MHz line isseen in emission at more negative velocities. These same examples are alsoobserved toward bright background continuum, many (perhaps all) show stimulatedemission, and 23 of these are coincident in on-sky position and velocity withH\textsc{ii}~radio recombination lines. To explain these remarkablecorrelations we propose that the 1720\,MHz stimulated emission originates inheated and compressed post-shock gas expanding away from a centralH\textsc{ii}~region, which collides with cooler and more diffuse gas hostingthe 1612\,MHz stimulated emission. The foreground gas dominates the spectrumdue to the bright central continuum, hence the expanding post-shock gas isblue-shifted relative to the stationary pre-shock gas. We employ non-LTEexcitation modelling to examine this scenario, and find that indeed FIRemission from warm dust adjacent to the H\textsc{ii}~region radiatively pumpsthe 1612 MHz line in the diffuse, cool gas ahead of the expanding shock front,while collisional pumping in the warm, dense shocked gas inverts the 1720 MHzline.

The hydroxyl satellite-line `flip' as a tracer of expanding HII regions