This paper presents an efficient and automated Overset Grid Assembly (OGA) method for the structured grid, and investigates high-order interpolation methods for inter-grid boundaries with the cell-centered finite difference method. Four enhancements are introduced: a hybrid hole-cutting approach integrates the efficiency of approximate hole-cutting and the accuracy and robustness of direct-cutting, effectively addressing challenges such as small gaps and thin cuts; an improved implicit hole boundary optimization method, incorporating quality comparison, can significantly reduce donor search workloads; an improved implicit interpolation cell cancellation algorithm minimizes overlap regions, particularly beneficial for high-order interpolation with large stencils; an algorithm for identifying and eliminating islands without using wall distances, effectively removes islands. The OGA results of a multi-element airfoil, a circular array of cylinders, multiple spheres, and a wing-pylon-store configuration indicate that the proposed method would be a suitable selection for multi-body problems, even in the presence of small gaps and thin geometries. Additionally, for high-order interpolation at inter-grid boundaries, this study presents an optimized interpolation method designed to minimize spectral property errors. Numerical results indicate that for periodic problems frequently crossing inter-grid boundaries, the optimized interpolation is more accurate than classical Lagrange interpolation and would be a suitable selection.