We describe the model configuration as well as simulations with the ocean-sea ice model ICON-O of the Max Planck Institute for Meteorology.ICON-O constitutes the ocean-sea ice component of the Earth-System Model ICON-ESM. It is an unstructured grid model that relies on structure-preserving Finite-Volume numerics. An advantage of unstructured grids is to provide a uniform coverage of the sphere but to allow also modifications to the uniform grid to become non-uniform and to concentrate resolution in any region of interest. This establishes a telescoping ability of the model to either resolvethe local circulation more accurately or to represent scales locally that can not be simulated globally. We demonstrate in this paper the fundamental capability of ICON-O to simulate essential features of global ocean dynamics at a uniform mesoscale eddy-resolving resolution of $\sim 10km$. Then we compare these results against a locally refined grid that achieves $\sim 10 km$ in the North Atlantic, while its resolution becomes increasingly coarser outside this focal region and smoothly approaches $\sim 80 km$. ICON-O in its locally refined configuration is able to reproduce the dynamics in the focal area over decadal time scales at a fraction of the computational costs of the uniform simulation. This telescoping technique provides an alternative to other regionalization approaches and opens new research directions.