We present the results of the first spectral line observations made with the Jodrell Bank Multi-Element Radio-Linked Interferometer Network (MERLIN). We describe the operation of MERLIN in its spectral line mode and the procedures used to analyse the data. We have observed the OH maser source W3OH at three ground-state transition frequencies, and have extended significantly the previous results on this source. In contrast with previous interpretations, we find that the masers are probably not directly associated with the compact H II region but instead are situated in clusters some of which are well separated from the H II region. We suggest that these clusters represent protostars whose formation was triggered by the shock wave associated with the compact H II region.