  The resurgence structure of the 2d $O(N)$ sigma model at large $N$ is studied
with a focus on an IR momentum cutoff scale $a$ that regularizes IR
singularities in the semiclassical expansion. Transseries expressions for
condensates and correlators are derived as series of the dynamical scale
$\Lambda$ (nonperturbative exponential) and coupling $\lambda_{\mu}$
renormalized at the momentum scale $\mu$. While there is no ambiguity when $a >
\Lambda$, we find for $a < \Lambda$ that the nonperturbative sectors have new
imaginary ambiguities besides the well-known renormalon ambiguity in the
perturbative sector. These ambiguities arise as a result of an analytic
continuation of transseries coefficients to small values of the IR cutoff $a$
below the dynamical scale $\Lambda$. We find that the imaginary ambiguities are
cancelled each other when we take all of them into account. By comparing the
semiclassical expansion with the transseries for the exact large-$N$ result, we
find that some ambiguities vanish in the $a \rightarrow 0$ limit and hence the
resurgence structure changes when going from the semiclassical expansion to the
exact result with no IR cutoff. An application of our approach to the ${\mathbb
C}P^{N-1}$ sigma model is also discussed. We find in the compactified model
with the ${\mathbb Z}_N$ twisted boundary condition that the resurgence
structure changes discontinuously as the compactification radius is varied.
