  We present the results of dust emission polarization measurements of
Ophiuchus-B (Oph-B) carried out using the Submillimetre Common-User Bolometer
Array 2 (SCUBA-2) camera with its associated polarimeter (POL-2) on the James
Clerk Maxwell Telescope (JCMT) in Hawaii. This work is part of the B-fields In
Star-forming Region Observations (BISTRO) survey initiated to understand the
role of magnetic fields in star formation for nearby star-forming molecular
clouds. We present a first look at the geometry and strength of magnetic fields
in Oph-B. The field geometry is traced over $\sim$0.2 pc, with clear detection
of both of the sub-clumps of Oph-B. The field pattern appears significantly
disordered in sub-clump Oph-B1. The field geometry in Oph-B2 is more ordered,
with a tendency to be along the major axis of the clump, parallel to the
filamentary structure within which it lies. The degree of polarization
decreases systematically towards the dense core material in the two sub-clumps.
The field lines in the lower density material along the periphery are smoothly
joined to the large scale magnetic fields probed by NIR polarization
observations. We estimated a magnetic field strength of 630$\pm$410 $\mu$G in
the Oph-B2 sub-clump using a Davis-Chandeasekhar-Fermi analysis. With this
magnetic field strength, we find a mass-to-flux ratio $\lambda$= 1.6$\pm$1.1,
which suggests that the Oph-B2 clump is slightly magnetically supercritical.
