2025ApJS..281...18G

Spectral morphological analysis is crucial for determining the fundamental physical properties of stars, particularly for rare and astrophysically important O-type stars. Due to heavy interstellar extinction in the optical range, most Galactic O stars remain obscured, making near-infrared (NIR) spectroscopy the only viable approach to achieve a reasonably complete sample of this stellar population and to extend current classification schemes. Our aim is to build a comprehensive spectral classification atlas of O-type stars in the NIR, providing robust morphological criteria to support and enhance future research on massive stars in heavily obscured environments. To this end, we obtained high-resolution (R ∼ 4000-11,000) NIR spectra for the majority of standard stars from the latest installment of the Galactic O-Star Spectroscopic Survey, using spectrographs at Gemini North and Las Campanas Observatory to ensure broad spectral coverage and high data quality. We present an atlas featuring spectral sequences for nine dwarfs (O4 to B0), nine giants (O3.5 to O9.5), and 20 supergiants (O2 to B0), systematically organised and accompanied by clear morphological criteria for subtype classification across the Y, J, H, and K bands (0.930-1.115, 1.160-1.290, 1.460-1.760, and 2.050-2.200 μm, respectively). This atlas provides a unified framework for reliable NIR classification of O-type stars and paves the way for detailed studies of massive stars in regions heavily affected by extinction.