MERLIN maps are presented of the OH 1665- and 1667-MHz masers in the circumstellar envelopes of the Mira variables U Her and R Cas, and the semiregular variable W Hya. For U Her, approximately 95 per cent of the OH maser emission comes from a partially filled thick shell, at radii between 50 and 135 au from the star. Across this shell, the logarithmic velocity gradient, ɛ, is in the range 0<ɛ<0.3. The OH masers of U Her have a core-halo structure. The strongest and most polarized emission occurs from ∼10 core components with angular sizes <0.03 arcsec. Weaker emission occurs from halo structures with angular sizes <1.0 arcsec. In R Cas, the OH masers have a clumpy distribution with no obvious centre of symmetry. The circumstellar envelope of this source may be partially disrupted by a binary companion star. Three possible envelope geometries are discussed for R Cas. In the preferred model, the masers are located within an incomplete thick shell at a mean radius of ∼110 au, with a logarithmic velocity gradient in the range 0<ɛ<1.0. For W Hya, MERLIN detected only the stronger emission from the front of the envelope, with a maximum detected OH extent of 60 au. For all three sources, the locations of the OH masers are consistent with standard models for the excitation of mainline OH, and with the estimated infrared mass-loss rates. We suggest that stellar mass loss from type I sources is intrinsically chaotic and clumpy.