  A famous example of gauge/gravity duality is the result that the entropy
density of strongly coupled ${\cal N}=4$ SYM in four dimensions for large N is
exactly 3/4 of the Stefan-Boltzmann limit. In this work, I revisit the massless
O(N) model in 2+1 dimensions, which is analytically solvable at finite
temperature $T$ for all couplings $\lambda$ in the large N limit. I find that
the entropy density monotonically decreases from the Stefan-Boltzmann limit at
$\lambda=0$ to exactly 4/5 of the Stefan-Boltzmann limit at $\lambda=\infty$.
Calculating the retarded energy-momentum tensor correlator in the scalar
channel at $\lambda=\infty$, I find that it has two logarithmic branch cuts
originating at $\omega=\pm 4 T \ln \frac{1+\sqrt{5}}{2}$, but no singularities
in the whole complex frequency plane. I show that the ratio 4/5 and the
location of the branch points both are universal within a large class of
bosonic CFTs in 2+1 dimensions.
