  OJ 287 is a well-studied binary black hole system, that occasionally exhibits
bright X-ray and optical flares. Here we present a detailed spectral study of
its second brightest X-ray flare observed during 2019-2020 using archival Swift
and NuSTAR observations along with ToO observations from AstroSat. The entire
flaring period is divided into three states, defined as low, intermediate, and
high states. The variation of hardness ratio (HR) with 0.3-10.0 keV integrated
flux suggests a \enquote{softer-when-brighter} behavior, as also previously
reported based on flux-index variations. Simultaneous high state X-ray spectra
obtained using Swift, NuSTAR and AstroSat are very steep with a power-law index
$>$2. A significant spectral change is observed in AstroSat-SXT and LAXPC
spectrum which is consistent with Swift-XRT and NuSTAR spectrum. Together,
optical-UV and X-ray spectrum during the high flux state, suggesting the
emergence of a new high BL Lacertae (HBL) component. We have modeled the
synchrotron peak with publicly available code named GAMERA for low,
intermediate, and high flux state. Our modeling suggests the need of high
magnetic field to explain the high state under the leptonic scenarios. Increase
in the magnetic field value inside the jet could be linked to the increase in
accretion rate as expected in the BH-disk impact scenario. The color-magnitude
diagram reveals a \enquote{bluer-when-brighter} spectral energy distribution
chromatism during the flaring period. Different chromatism or no chromatism at
various occasions suggests a complex origin of optical emission, which is
believed to be produced by disc impact or through synchrotron emission in the
jet.
