  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.
