Low-mass extremely metal-poor (EMP) stars formed from a mixture of material of the primordial nucleosynthesis and matter ejected from the first supernovae. The chemical composition of EMP stars, especially those still on the main sequence, holds crucial information on the properties of the first stars, the first supernovae and the early chemical enrichment of the Universe. We discovered the iron-poor dwarf star SDSS J0815+4729 (Aguado et al. 2018a, ApJ Letters) using the OSIRIS spectrograph at the 10.4m-GTC telescope in La Palma (Canary Islands, Spain), revealing already the huge C enhancement in this primitive star with a million times less iron than the Sun. We have recently acquired high-resolution spectroscopy with HIRES at the 10m-KeckI telescope, uncovering the unique abundance pattern of J0815+4729 (González Hernández et al. 2020, ApJ Letters). We derive [Fe/H] = -5.5 and detect the near-IR OI triplet for the first time in an ultra metal-poor star, confirming the extreme CNO abundances of J0815+4729 with ratios [X/Fe] > 4.