  Aims. We present and analyse late-time observations of the type-Ib supernova
with possible pre-supernova progenitor detection, iPTF13bvn, taken at $\sim$300
days after the explosion, and discuss these in the context of constraints on
the supernova's progenitor. Previous studies have proposed two possible natures
for the progenitor of the supernova, i.e. a massive Wolf-Rayet star or a
lower-mass star in close binary system. Methods. Our observations show that the
supernova has entered the nebular phase, with the spectrum dominated by
Mg~I]$\lambda\lambda$4571, [O~I]$\lambda\lambda$6300, 6364, and
[Ca~II]$\lambda\lambda$7291, 7324 emission lines. We measured the emission line
fluxes to estimate the core oxygen mass and compare the [O~I]/[Ca~II] line
ratio with other supernovae. Results. The core oxygen mass of the supernova
progenitor was estimated to be $\lesssim$0.7 M$_\odot$, which implies initial
progenitor mass not exceeding $\sim$15 -- 17 M$_\odot$. Since the derived mass
is too small for a single star to become a Wolf-Rayet star, this result lends
more support to the binary nature of the progenitor star of iPTF13bvn. The
comparison of [O~I]/[Ca~II] line ratio with other supernovae also shows that
iPTF13bvn appears to be in close association with the lower-mass progenitors of
stripped-envelope and type-II supernovae.
