Object-Oriented programming and the reuse of classes and methods, when practised on a large scale by communities of programmers and designers sharing software components, will lead to the creation of very large class collections. In such a context, facilities for querying a class collection become important. This paper presents an object-oriented data model, Xos, which can be used to logically organize and describe a class collection in a form suitable for querying. An extended example based on modelling C++ is given.