  We report detection of a very bright X-ray-UV-optical outburst of OJ 287 in
April-June 2020; the second brightest since the beginning of our Swift
multi-year monitoring in late 2015. It is shown that the outburst is
predominantly powered by jet emission. Optical-UV-X-rays are closely
correlated, and the low-energy part of the XMM-Newton spectrum displays an
exceptionally soft emission component consistent with a synchrotron origin. A
much harder X-ray powerlaw component (Gamma-x = 2.4, still relatively steep
when compared to expectations from inverse-Compton models) is detected out to
70 keV by NuSTAR. We find evidence for reprocessing around the Fe region,
consistent with an absorption line. If confirmed, it implies matter in outflow
at approx 0.1c. The multi-year Swift lightcurve shows multiple episodes of
flaring or dipping with a total amplitude of variability of a factor of 10 in
X-rays, and 15 in the optical-UV. The 2020 outburst observations are consistent
with an after-flare predicted by the binary black hole model of OJ 287, where
the disk impact of the secondary black hole triggers time-delayed accretion and
jet activity of the primary black hole.
