The OSInstance class is the in-memory representation of an optimization instance and is a key class for users of the OS project. This class has an API defined by a collection of get() methods for extracting various components (such as bounds and coefficients) from a problem instance, a collection of set() methods for modifying or generating an optimization instance, and a collection of calculate() methods for function, gradient, and Hessian evaluations. See Section 8. We now describe how to create an OSInstance object and the close relationship between the OSiL schema and the OSInstance class.