We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a Chandrasekhar-Fermi analysis using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We demonstrate methods for combining BISTRO data with previous SCUBA-2 and HARP observations in order to perform a Chandrasekhar-Fermi analysis. We find a plane-of-sky magnetic field strength in OMC 1 of Bpos = 6.4 ± 2.1 mG, determined using a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of (1.6 ± 1.1) × 10-7 J m-3 in OMC 1, comparable both to the energy density in the gravitational field of the Orion BN/KL-S system (∼ 10−7 Jm−3), and to the energy density in the Orion BN/KL outflow (∼ 10−7 Jm−3). We find that neither the Alfv ?en velocity in OMC 1 nor the velocity of the super-Alfv ?enic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically- symmetric magnetic field by the gravitational attraction of the BN/KL and S clumps. We further suggest that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.