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The Gamma-ray BL Lac object OJ 287 is known to exhibit inner-parsec"jet-wobbling", high degrees of variability at all wavelengths andquasi-stationary features including an apparent (~100 deg) position anglechange in projection on the sky plane. Sub-50 micro-arcsecond resolution 86 GHzobservations with the global mm-VLBI array (GMVA) supplement ongoingmulti-frequency VLBI blazar monitoring at lower frequencies. Using these mapstogether with cm/mm total intensity and Gamma-ray observations from Fermi/LATfrom 2008-2014, we aimed to determine the location of Gamma-ray emission and toexplain the inner-mas structural changes. Observations with the GMVA offerapproximately double the angular resolution compared with 43 GHz VLBAobservations and allow us to observe above the synchrotron self-absorption peakfrequency. The jet was spectrally decomposed at multiple locations along thejet. From this we derived estimates of the magnetic field. How the fielddecreases down the jet allowed an estimate of the distance to the jet apex andan estimate of the magnetic field strength at the jet apex and in the broadline region. Combined with accurate kinematics we attempt to locate the site ofGamma-ray activity, radio flares and spectral changes. Strong Gamma-ray flaresappeared to originate from either the "core" region, a downstream stationaryfeature, or both, with Gamma-ray activity significantly correlated with radioflaring in the downstream quasi-stationary feature. Magnetic field estimateswere determined at multiple locations along the jet, with the magnetic fieldfound to be >1.6 G in the "core" and >0.4 G in the downstream quasi-stationaryfeature. We therefore found upper limits on the location of the "core" as >6.0pc from the jet apex and determined an upper limit on the magnetic field nearthe jet base of the order of thousands of Gauss.

Location of Gamma-ray emission and magnetic field strengths in OJ 287