  The aim of this paper is to establish $W^2_p$ estimate for non-divergence
form second-order elliptic equations with the oblique derivative boundary
condition in domains with small Lipschitz constants. Our result generalizes
those in [14, 15], which work for $C^{1,\alpha}$ domains with $\alpha > 1-1/p$.
As an application, we also obtain a solvability result. An extension to fully
nonlinear elliptic equations with the oblique derivative boundary condition is
also discussed.
