OH megamaser emission is generally associated with starburst activity in the centers of Ultraluminous IR galaxies. Detailed observations of the maser emission therefore provides a method to investigate the structure and kinematics of the central regions in such galaxies. In this talk I will concentrate on high resolution observations of the OH megamaser galaxy IIIZw35. In this source the masers trace a ring structure. Modeling of the maser emission illustrates the importance of clumpiness in the OH maser medium, and also explains the bright maser spots often observed by VLBI in OH megamaser sources.