ARP 220 is the prototype far-infrared ultraluminous galaxy, and the origin of its luminosity-a burst of massive star formation or a quasar obscured by a layer of dense gas and dust-has been the subject of much debate(1,2) It also contains the prototypical OH megamaser(3)-an extremely luminous version of masers (microwave lasers) commonly found in our own galaxy. It has been thought that the dense gas in the inner few hundred parsecs of megamaser galaxies acts as a low-gain masing screen, pumped by the far-infrared radiation, which amplifies background continuum emission from the nuclear regions(4-6). Here we show, using new very-long-baseline interferometry observations, that the OH line peak in Arp 220 originates in a structure less than or equal to 1 pc across, positionally aligned with a weak continuum feature, and that most of the emission originates on scales of less than or equal to 10 pc. These results imply that the maser is physically 10-100 times smaller than previously thought(5,6), strongly suggesting that much of the far-infrared radiation from Arp 220 arises in a very small region, possibly a dense molecular torus, surrounding a quasar nucleus.