  OH (1720 MHz) masers serve as indicators of SNR - molecular cloud interaction
sites. These masers are collisionally excited in warm (50-100K) shocked gas
with densities of order 1e5 cm^-3 when the OH column density is in the range
1e16-1e17 cm^-2. Here I present excitation calculations which show that when
the OH column density exceeds 1e17 cm^-2 at similar densities and temperatures,
the inversion of the 1720 MHz line switches off and instead the 6049 MHz
transition in the first excited rotational state of OH becomes inverted. This
line may serve as a complementary signal of warm, shocked gas when the OH
column density is large.
