We present MERLIN observations of OH maser and radio continuum emission from the Ultra Luminous IR Galaxy Markarian 231. The 1665- and 1667-MHz transitions have a combined velocity extent of 720 km/s and show a similar position-velocity structure including a gradient of 1.7 km/s/pc from NW to SE along the 420-pc major axis, steeper in the inner few tens of pc. The maser distribution is modelled as a torus rotating about an axis inclined at ~45deg. We estimate the enclosed mass density to be 320(90) Msun in a flattened distribution, including a central unresolved mass of </=8E+06 Msun. All the maser emission is projected against a region with a radio continuum brightness temperature >/=1E+05 K, giving a maser gain of </=2.2. The 1667:1665-MHz line ratio is close to the LTE ratio of 1.8 consistent with radiatively pumped, unsaturated masers. The size of individual masing regions is in the range 0.25-4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273 where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification as a Seyfert 1. Most of the radio continuum emission on 50-500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north-south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped.
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