  We present a model for the OH megamaser emission in the starburst galaxy
IIIZw35. The observed diffuse and compact OH maser components in this source
are explained by a single phase of unsaturated clumpy gas distributed in a thin
ring structure and amplifying background continuum. We emphasize the importance
of clumpiness in the OH masing medium, an effect that has not been fully
appreciated previously.
  The model explains why multiple bright spots are seen only at the ring
tangents while smoother emission is found elsewhere. Both the observed velocity
gradients and the line to continuum ratios around the ring enquire a geometry
where most of the seed photons come from a continuum emission which lies
outside the OH ring. To explain both the OH and continuum brightness, free-free
absorbing gas is required along the ring axis to partially absorb the far side
of the ring. It is proposed that the required geometry arises from an inwardly
propagating ring of starburst activity.
