  We report the first detection of a hydroxyl radical (OH) emission signature
in the planetary atmosphere outside the solar system, in this case, in the
day-side of WASP-33b. We analyze high-resolution near-infrared emission spectra
of WASP-33b taken using the InfraRed Doppler spectrograph on the 8.2-m Subaru
telescope. The telluric and stellar lines are removed using a de-trending
algorithm, SysRem. The residuals are then cross-correlated with OH and H$_{2}$O
planetary spectrum templates produced using several different line-lists. We
check and confirm the accuracy of OH line-lists by cross-correlating with the
spectrum of GJ 436. As a result, we detect the emission signature of OH at
$K_\mathrm{p}$ of 230.9$^{+6.9}_{-7.4}$ km s$^{-1}$ and $v_{\mathrm{sys}}$ of
$-$0.3$^{+5.3}_{-5.6}$ km s$^{-1}$ with S/N of 5.4 and significance of
5.5$\sigma$. Additionally, we marginally detect H$_{2}$O emission in the H-band
with S/N of 4.0 and significance of 5.2$\sigma$ using the POKAZATEL line-list.
However, no significant signal is detected using the HITEMP 2010, which might
be due to differences in line positions and strengths, as well as the
incompleteness of the line-lists. Nonetheless, this marginal detection is
consistent with the prediction that H$_{2}$O is mostly thermally dissociated in
the upper atmosphere of the ultra-hot Jupiters. Therefore, along with CO, OH is
expected to be one of the most abundant O-bearing molecules in the day-side
atmosphere of ultra-hot Jupiters and should be considered when studying their
atmosphere.
