Multi‐Element Radio‐Linked Interferometer Network polarization measurements of the OH 18‐cm masers in ON1 are presented. OH maser emission is detected in all four ground state lines. The bulk of the masers lie in an elongated distribution covering 2 arcsec at position angle 130°, near the southern edge of the ultracompact H ii region. The maser distribution is orthogonal to the bipolar outflow seen in HCO+ at position angle (PA) 44°. An isolated group of 1720‐MHz masers is also seen 6 arcsec to the east. The magnetic field deduced from Zeeman splitting of the OH maser lines shows a large‐scale order, with field values ranging from −0.4 to −4.6 mG.

OH masers are sensitive probes of the kinematics and physical conditions, and give unique information on the magnetic field through their polarization. Zeeman splitting of the OH lines can give the magnetic field strength and direction. Observing OH masers with MERLIN we studied the bipolar outflow in the star-forming region ON1, which hosts one of the earliest known ultra-compact (UC) HII regions. The strongest masers lie near the southern edge of the UCHII region in an elongated distribution. The maser distribution is orthogonal to the bipolar outflow seen in HCO + , suggesting that the OH masers may be embedded in a molecular disk or torus around a young B0.3 star, most likely tracing a shock front. An isolated group of 1720-MHz masers is also seen to the East. The magnetic field deduced from Zeeman splitting of the OH maser lines shows a large-scale order, with field values ranging from -0.4 to -4.6 mG. These results add to the growing body of evidence for OH masers associated with molecular disks or tori at the centre of bipolar outflow from massive young stars, and for a significant role played by the magnetic field in generating or channeling the bipolar outflow. Further details are presented by Nammahachak et al. 2006.