  From targeted observations of ground-state OH masers towards 702 Multibeam
(MMB) survey 6.7-GHz methanol masers, between Galactic longitudes 186$^{\circ}$
through the Galactic centre to 20$^{\circ}$, made as part of the `MAGMO'
project, we present the physical and polarisation properties of the 1720-MHz OH
maser transition, including the identification of Zeeman pairs. We present 10
new and 23 previously catalogued 1720-MHz OH maser sources detected towards
star formation regions. In addition, we also detected 16 1720-MHz OH masers
associated with supernova remnants and two sites of diffuse OH emission.
Towards the 33 star formation masers, we identify 44 Zeeman pairs, implying
magnetic field strengths ranging from $-$11.4 to $+$13.2 mG, and a median
magnetic field strength of $|B_{LOS}|$ $\sim$ 6 mG. With limited statistics, we
present the in-situ magnetic field orientation of the masers and the Galactic
magnetic field distribution revealed by the 1720-MHz transition. We also
examine the association statistics of 1720-MHz OH SFR masers with other
ground-state OH masers, excited-state OH masers, class I and class II methanol
masers and water masers, and compare maser positions with mid-infrared images
of the parent star forming regions. Of the 33 1720-MHz star formation masers,
ten are offset from their central exciting sources, and appear to be associated
with outflow activity.
