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We report the first detection of a hydroxyl radical (OH) emission signaturein the planetary atmosphere outside the solar system, in this case, in theday-side of WASP-33b. We analyze high-resolution near-infrared emission spectraof WASP-33b taken using the InfraRed Doppler spectrograph on the 8.2-m Subarutelescope. The telluric and stellar lines are removed using a de-trendingalgorithm, SysRem. The residuals are then cross-correlated with OH and H$_{2}$Oplanetary spectrum templates produced using several different line-lists. Wecheck and confirm the accuracy of OH line-lists by cross-correlating with thespectrum 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 of5.5$\sigma$. Additionally, we marginally detect H$_{2}$O emission in the H-bandwith 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 mightbe due to differences in line positions and strengths, as well as theincompleteness of the line-lists. Nonetheless, this marginal detection isconsistent with the prediction that H$_{2}$O is mostly thermally dissociated inthe upper atmosphere of the ultra-hot Jupiters. Therefore, along with CO, OH isexpected to be one of the most abundant O-bearing molecules in the day-sideatmosphere of ultra-hot Jupiters and should be considered when studying theiratmosphere.

First Detection of Hydroxyl Radical Emission from an Exoplanet Atmosphere: High-dispersion Characterization of WASP-33b using Subaru/IRD