We present an analysis of high-resolution Keck/HIRES spectroscopic observations of J0815+4729, an extremely carbon-enhanced, iron-poor dwarf star. These high-quality data allow us to derive a metallicity of [Fe/H]$=-5.49{pm}0.14$ from the three strongest ion{Fe}{1} lines and to measure a high [Ca/Fe]~$=0.75{pm}0.14$. The large carbon abundance of A(C)~$=7.43{pm}0.17$ (or [C/Fe]~$sim 4.49{pm}0.11$) places this star in the upper boundary of the low-carbon band in the A(C)-[Fe/H] diagram, suggesting no contamination from a binary AGB companion. We detect the oxygen triplet at 777nm for the first time in an ultra-metal poor star, indicating a large oxygen-to-iron abundance ratio of [O/Fe]~$=4.03{pm}0.12$ (A(O)~$=7.23{pm}0.14$), significantly higher than the previously most metal-poor dwarf J2209-0028 with an oxygen triplet detection with [O/Fe]~$sim2.2$~dex at [Fe/H]~$sim -3.9$. Nitrogen is also dramatically enhanced with (A(N)~$=6.75{pm}0.08$) and an abundance ratio [N/Fe]~$sim 4.41{pm}0.08$. We also detect Ca, Na and Mg, while provide upper limits for eight other elements. The abundance pattern of J0815+4729 resembles that of HE~1327-2326, indicating that both are second-generation stars contaminated by a $sim 21-27$~msun~single, zero-metallicity low-energy supernova with very little mixing and substantial fallback. The absence of lithium implies an upper-limit abundance A(Li)~$<1.3$~dex, about 0.7~dex below the detected Li abundance in J0023+0307 which has a similar metallicity, exacerbating the cosmological lithium problem.
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