  I derive an exact integral expression for the ratio of shear viscosity over
entropy density $\frac{\eta}{s}$ for the massless (critical) O(N) model at
large N with quartic interactions. The calculation is set up and performed
entirely from the field theory side using a non-perturbative resummation scheme
that captures all contributions to leading order in large N. In 2+1d,
$\frac{\eta}{s}$ is evaluated numerically at all values of the coupling. For
infinite coupling, I find $\frac{\eta}{s}\simeq 0.23(1)\times N$. I show that
this strong coupling result for the viscosity is universal for a large class of
interacting bosonic O(N) models.
