Our work focuses on high-accuracy spectral modeling in NLTE, and the determination of chemical abundances for the oldest known stars, providing crucial insights into the early universe and nucleosynthesis processes. Utilizing state-of-the-art spectroscopic techniques, we have analyzed high-resolution observations of the hyper metal-poor star J0815+4729 to derive its precise elemental abundances, including the elements carbon and oxygen. We discuss the implications of our findings on our understanding of the formation and evolution of the oldest stars. In addition, we are constructing a comprehensive NLTE synthetic spectral library that spans from 0.2 to 3 μm in wavelength, which will be made accessible to the public. This spectral library will make a significant impact to large spectroscopy surveys such as APOGEE, DESI, LAMOST, 4MOST, and others.