  We investigate the mechanisms by which a particle bed modifies the
oscillatory boundary layer. We perform Euler-Lagrange simulations of an OBL
over a particle bed at Reynolds numbers $\mathrm{Re}_{\delta}= 200$, 400, and
800, density ratio $\rho_p/\rho_f = 2.65$, maximum Shields numbers ranging from
$1.42 \times 10^{-2}$ to $7.40 \times 10^{-2}$, Galileo number $\mathrm{Ga} =
51.9$, Stokes number ranging from 3.99 to 15.97, and Kuelegan-Carpenter number
ranging from $1.7 \times 10^3$ to $6.8 \times 10^3$, and showed large
modulation to the underlying boundary layer. We show two mechanisms of
modification to the oscillatory boundary layer: (I) the permeability of the bed
allows fluid flow to penetrate into the bed, introducing a slip velocity at the
bed-fluid interface, an inflection point and the expansion of the boundary
layer, and (II) for $\mathrm{Re}_{\delta} = 400$, and 800, particle motion
creates an evolving bedform. For $\mathrm{Re}_{\delta} = 200$, particles are
nearly motionless and the permeability of the bed drives the alteration of the
velocity profile. The initial bed form drives velocity fluctuations not seen
for flow over a smooth, impermeable wall. For Reynolds 400 and 800 particle
momentum is associated with the peak velocity fluctuations, indicating that the
particle feedback force contributes to velocity fluctuations. We show that the
coefficient of friction, computed using the bed shear stress, is decreased by
nearly half for Reynolds 200 and 400, and nearly an eighth for Reynolds 800.
