  We report a strong soft X-ray excess in the BL--Lacartae $\gamma$-ray blazar
OJ~287 during the long exposure in May 2015, amongst two of the latest \xmm~{}
observations performed in May 2015 and 2018. In case of May 2015 observation, a
logparabola model fits the EPIC-pn data well while a logparabola plus powerlaw
describes the overall simultaneous optical to X-ray spectra, suggesting the
excess as the synchrotron tail. This interpretation, however, is inconsistent
with the observed spectral break between near-infrared and optical spectra,
attributed to standard disk around a supermassive black hole (SMBH). Based on
this, we considered two commonly invoked accretion disk based models in AGNs to
explain the soft excess: the cool Comptonization component in the accretion
disk and the blurred reflection from the partially ionized accretion disk. We
found that both cool Comptonization and blurred reflection models provide
equally good fit to the data and favor a super-heavy SMBH of mass $\sim
10^{10}~M_\odot$. Further investigation of about a month long simultaneous
X-ray and UV pointing observations revealed a delayed UV emission with respect
to the 1.5-10 keV band, favoring X-ray reprocessing phenomenon as the dominant
mechanism. The results suggest that the soft excess is probably caused by
strong light bending close to the SMBH. The detected soft excess in 2015 data
and its disappearance in 2018 data is also consistent with the presence of
accretion disk emission, inferred from the NIR-optical spectral} break between
May 2013 to March 2016.
