  We present a comprehensive analysis of all XMM-Newton spectra of OJ 287
spanning 15 years of X-ray spectroscopy of this bright blazar. We also report
the latest results from our dedicated Swift UVOT and XRT monitoring of OJ 287
which started in 2015, along with all earlier public Swift data since 2005.
During this time interval, OJ 287 was caught in extreme minima and outburst
states. Its X-ray spectrum is highly variable and encompasses all states seen
in blazars from very flat to exceptionally steep. The spectrum can be
decomposed into three spectral components: Inverse Compton (IC) emission
dominant at low-states, super-soft synchrotron emission which becomes
increasingly dominant as OJ 287 brightens, and an intermediately-soft
(Gamma_x=2.2) additional component seen at outburst. This last component
extends beyond 10 keV and plausibly represents either a second synchrotron/IC
component and/or a temporary disk corona of the primary supermassive black hole
(SMBH). Our 2018 XMM-Newton observation, quasi-simultaneous with the Event
Horizon Telescope observation of OJ 287, is well described by a two-component
model with a hard IC component of Gamma_x=1.5 and a soft synchrotron component.
Low-state spectra limit any long-lived accretion disk/corona contribution in
X-rays to a very low value of L_x/L_Edd < 5.6 times 10^(-4) (for M_(BH,
primary) = 1.8 times 10^10 M_sun). Some implications for the binary SMBH model
of OJ 287 are discussed.
