ICON-O is a core component of the German natESM.
ICON-O is a global ocean general circulation model that works on unstructured grids. It rests on novel numerical techniques that belong to the class of structure-preserving finite Volume methods. It builds on the same ICON infrastructure, but applies the Boussinesq and hydrostatic approximation and includes a sea-ice model. The state vector consists of horizontal velocity, the oceanic tracers potential temperature and salinity, as well as the surface elevation. The primitive equations are solved on the triangular ICON grid with an Arakawa C-type staggering that places tracers at the circumcenter of a triangular cell and the normal component of the velocity vector at the midpoint of the cells edge.
The atmosphere model ICON-A and the ocean model ICON-O have been developed jointly. A main accomplishment of the joint development is that the two models share the same technical infrastructure that covers in particular High-Performance-Computing aspects.
The software ICON in general is now open source under BSD-3C license.
More information, references, model description about ICON-O here and here and testing of the model performance here. The ICON documentation is filed on an ICON-wiki.
If you want to get started, you have to get in touch with the developers. There are also yearly Numerical Model Training Courses - more information.