Context. 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 (approximate to 100 degrees) position-angle change in projection on the sky plane.

Aims. 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 aim to determine the location of gamma-ray emission and to explain the inner-mas structural changes.

Methods. Observations with the GMVA offer approximately double the angular resolution compared with 43 GHz VLBA observations and enable us to observe above the synchrotron self-absorption peak frequency. Fermi-LAT gamma-ray data were reduced and analysed. The jet was spectrally decomposed at multiple locations along the jet. From this, we could derive estimates of the magnetic field using equipartition and synchrotron self-absorption arguments. How the field decreases down the jet provided 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.

Results. Strong gamma-ray flares appeared to originate from either the so-called 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 >= 1.6G in the core and