  OAO 1657-415 is an accreting X-ray pulsar with a high mass companion that has
been observed by several telescopes over the years, in different orbital
phases. Back in 1999, observations performed with Beppo-SAX lead to the
detection of a cyclotron-resonant-scattering feature, which has not been found
again with any other instrument. A recent NuSTAR X-ray observation, performed
during the brightest phase of the source, allows us to perform sensitive
searches for cyclotron-resonant-scattering features in the hard X-ray spectrum
of the source. We aim to characterise the source by means of temporal and
spectral X-ray analysis, and to confidently search for the presence of
cyclotron-resonant-scattering features. The observation was divided into four
time intervals in order to characterise each one. Several timing analysis tools
were used to obtain the pulse of the neutron star, and the light curves folded
into the time intervals. The NuSTAR spectrum in the energy range 3-79 keV was
used, which was modelled with a power-law continuum emission model with a
high-energy cutoff. We find the pulsations associated with the source in the
full observation, which are shifted due to the orbital Doppler effect. We show
evidence that a cyclotron line at $35.6 \pm 2.5$ keV is present in the
spectrum. We use this energy to estimate the dipolar magnetic field at the
pulsar surface to be $4.0\pm 0.2\,\times\,10^{12}$ G. We further estimate a
lower limit in the distance to OAO 1657-415 of $\simeq 1$ kpc. And we also find
a possible positive correlation between the luminosity and the energy
associated with the cyclotron line. We conclude that the cyclotron line at
$35.6\pm2.5$ keV is the same as that detected by Beppo-SAX. Our detection has a
significance of $\sim$ 3.4$\sigma$.
