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OAO 1657-415 is an eclipsing X-ray binary wind-fed pulsar that has exhibitedsmooth spin-up/spin-down episodes and has undergone several torque reversalsthroughout its long history of observation. We present a frequency historyspanning nearly 19 years of observations from the Burst and Transient SourceExperiment (CGRO/BATSE) and from the Gamma-Ray Burst Monitor (Fermi/GBM). Theanalysis suggests two modes of accretion: one resulting in steady spin-upduring which we believe a stable accretion disk is present and one that resultsin what appears to be a random walk in spin frequency where an unstableaccretion disk forms alternating in direction ("flip flop"). Orbital elementsof the pulsar system are determined at several intervals throughout thishistory. With these ephemerides, statistically significant orbital decay($\dot{P}/P =(-3.40 \pm0.15)\times10^{-6} \mathrm{yr^{-1}}$) is establishedsuggesting a transition between wind-fed and disk-mediated accretion.

Orbital Decay and Evidence of Disk Formation in the X-ray Binary Pulsar OAO 1657-415