We present the results of MERLIN polarization mapping of OH masers at 1665 and 1667 MHz towards the Cepheus A star-forming region. The maser emission is spread over a region of 6 arcsec by 10 arcsec, twice the extent previously detected. In contrast to the 22 GHz water masers, the OH masers associated with H II regions show neither clear velocity gradients nor regular structures. We identified ten Zeeman pairs which imply a magnetic field strength along the line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the arcsecond scale, pointing towards us in the west and away from us in the east side. The linearly polarized components, detected for the first time, show regularities in the polarization position angles depending on their position. The electric vectors of OH masers observed towards the outer parts of H II regions are consistent with the interstellar magnetic field orientation, while those seen towards the centres of H II regions are parallel to the radio-jets. A Zeeman quartet inside a southern H II region has now been monitored for 25 years; we confirm that the magnetic field decays monotonically over that period.
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