Results of NICER monitoring of OAO 1657–415 over three different orbits shown by three colors. From top to bottom are the global absorption column density (N_H1; in units of 10²² cm⁻²), spectral index Γ, normalization of the spectral index (Γ_norm; in photons keV⁻¹ cm⁻² s⁻¹ at 1 keV), local absorption column density (N_H2; in units of 10²² cm⁻²), and EW of the iron line (in keV). The squares in the top panel indicate the values of N_H1 where the errors could not be determined and were frozen to its best-fit values.