High-resolution spectroscopy is one of the most successful methods to characterise exoplanet atmospheres. Due to the planetary orbital motion, the planetary lines can be distinguished from the telluric and stellar lines. By comparing hundreds/thousands of unique absorption/emission lines to model templates, the atomic/molecular signatures can be unambiguously identified and detected at high significance. I will share our recent detection of OH on the day side of WASP-33b using the InfraRed Doppler instrument on the Subaru telescope. Although it can be found in the atmosphere of Earth, Venus, Mars, and Saturn’s magnetosphere, this is the first known OH molecular signature in the atmosphere of an exoplanet. We also found evidence of weak H₂O emission which indicates a thermal-dissociation in the upper-atmosphere. Along with CO, OH is expected to be one of the most dominant O-bearing molecules in the atmosphere of ultra hot Jupiters. Additionally, we found that the strength of the H₂O signal depends on the line-list used in producing the spectral template.