  (abridged) The role of magnetic field in the shaping of Planetary Nebulae
(PNe), either directly or indirectly after being enhanced by binary
interaction, has long been a topic of debate. Large scale magnetic fields
around pre-PNe have been inferred from polarisation observations of masers.
However, because masers probe very specific regions, it is still unclear if the
maser results are representative of the intrinsic magnetic field in the
circumstellar envelope (CSE). Molecular line polarisation can provide important
information about the magnetic field. A comparison between the field morphology
determined from maser observations and that observed in the more diffuse CO
gas, can reveal if the two tracers probe the same magnetic field. We compare
observations taken with ALMA of molecular line polarisation around the
post-Asymptotic Giant Branch)/pre-PNe star OH~17.7-2.0 with previous
observations of polarisation in the 1612~MHz OH maser region. We detect
CO~$J=2-1$ molecular line polarisation at a level of $\sim4\%$ that displays an
ordered linear polarisation structure. We find that, correcting for Faraday
rotation of the OH~maser linear polarisation vectors, the OH and CO linearly
polarised emission trace the same large scale magnetic field. A structure
function analysis of the CO linear polarisation reveals a plane-of-the-sky
magnetic field strength of $B_\perp\sim1$~mG in the CO region, consistent with
previous OH Zeeman observations. The consistency of the ALMA CO molecular line
polarisation with maser observations indicate that both can be used to
determine the magnetic field in CSEs. The existence of a strong, ordered,
magnetic-field around OH 17.7-2.0 indicates that magnetic fields are likely
involved in the formation of this bipolar pre-PNe.
