  The Gamma-ray BL Lac object OJ 287 is known to exhibit inner-parsec
"jet-wobbling", high degrees of variability at all wavelengths and
quasi-stationary features including an apparent (~100 deg) position angle
change in projection on the sky plane. Sub-50 micro-arcsecond resolution 86 GHz
observations with the global mm-VLBI array (GMVA) supplement ongoing
multi-frequency VLBI blazar monitoring at lower frequencies. Using these maps
together with cm/mm total intensity and Gamma-ray observations from Fermi/LAT
from 2008-2014, we aimed to determine the location of Gamma-ray emission and to
explain the inner-mas structural changes. Observations with the GMVA offer
approximately double the angular resolution compared with 43 GHz VLBA
observations and allow us to observe above the synchrotron self-absorption peak
frequency. The jet was spectrally decomposed at multiple locations along the
jet. From this we derived estimates of the magnetic field. How the field
decreases down the jet allowed an estimate of the distance to the jet apex and
an estimate of the magnetic field strength at the jet apex and in the broad
line region. Combined with accurate kinematics we attempt to locate the site of
Gamma-ray activity, radio flares and spectral changes. Strong Gamma-ray flares
appeared to originate from either the "core" region, a downstream stationary
feature, or both, with Gamma-ray activity significantly correlated with radio
flaring in the downstream quasi-stationary feature. Magnetic field estimates
were determined at multiple locations along the jet, with the magnetic field
found to be >1.6 G in the "core" and >0.4 G in the downstream quasi-stationary
feature. We therefore found upper limits on the location of the "core" as >6.0
pc from the jet apex and determined an upper limit on the magnetic field near
the jet base of the order of thousands of Gauss.
