The physical conditions of kinetic temperature T-K, dust temperature T-d, OH number density n(OH), H-2 number density n(H-2) velocity fields and microturbulence, in OH-containing clouds in OH megamaser galaxies, have been investigated using Accelerated Lambda Iteration (ALI). Compact (3 pc) clouds, containing dust with T-d > T-K > 40 K to 50 K, with n(H-2) between a few times 10(3) and a few times 10(4) cm(-3) and n(OH) similar to 10(-4) cm(-3), are radiatively pumped to yield dominant 1667 MHz emission, with 1667/1665 flux ratios covering the full range of those observed. Larger clouds, between 25-50 pc and 300 pc, tend to show weaker 18 cm emission or 18 cm absorption for the same OH column densities of similar to 10(15) cm(-2) These clouds yield 6 GHz absorption, as observed in Arp 220. Velocity fields are influential in determining the emission and absorption properties of clouds via effects of FIR line overlap. Microturbulence does not however play an important role in determining OH megamaser characteristics. A failure of the present work is the inability to model absorption at 4765 MHz, observed in a number of megamaser galaxies. Using the results of similar to 2000 models, it is possible to estimate physical conditions in several zones in Arp 220, IRAS 17208-0014 and III Zw 35, using the observed 18 cm line ratios, with data for the satellite lines. Models also predict that OH-absorption galaxies, showing absorption in 18 cm and 5 cm main lines, may frequently be accompanied by weak 1720 MHz emission. 1612 MHz emission may be diagnostic of the presence of larger clouds (50-100 pc). These and other predictions may be directly investigated by observation. Multi-frequency, phase referenced interferometric data, including continuum maps, are essential for significant progress in this field. Our results are consistent with a model in which OH megamaser galaxies may evolve into OH absorption galaxies, through OH-containing clouds coalescing, becoming depleted in OH and suffering a reduction in dust temperature to equal or below a kinetic temperature of 40 K to 45 K.
