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This work presents a comprehensive timing and spectral analysis of high-massX-ray binary pulsar, OAO 1657-415 by using the observation made with NuclearSpectroscopic Telescope Array (NuSTAR) on June 2019. During this observation,OAO 1657-415 exhibited X-ray variability by a factor of about 3. X-raypulsations at 37.03322(14) s were observed up to 70 keV. OAO 1657-415 wasundergoing a spin-down phase with $\dot{P} = 9(1) \times 10^{-8}$ s s$^{-1}$.This is an order of about 100 larger than the long-term spin-up rate. The pulseprofile evolved marginally during the observation. We have discussed thelong-term pulse period history of the source spanning a time-base of 43 years,including the latest Fermi/GBM data. The 3-70 keV source spectrum is describedby a partially covered cutoff power-law, an Fe K$_{\alpha}$ line at 6.4 keV anda Compton shoulder at 6.3 keV. We report the presence of a cyclotron absorptionfeature around 40 keV, which is indicative of a surface magnetic field strengthof $3.59 \pm 0.06 \ (1 + z)^{-1} \times 10^{12}$ and $3.29_{-0.22}^{+0.23} \ (1+ z)^{-1} \times 10^{12}$ G. This work shows the first robust presence ofcyclotron absorption feature in the source.

Timing and Spectral Analysis of HMXB OAO 1657-415 with NuSTAR