oomph-lib: An object-oriented multi-physics finite-element library. This paper discusses certain aspects of the design and implementation of oomph-lib, an object-oriented multi-physics finite-element library, available as open-source software at http://www.oomph-lib.org. The main aim of the library is to provide an environment that facilitates the robust, adaptive solution of multi-physics problems by monolithic discretisations, while maximising the potential for code re-use. This is achieved by the extensive use of object-oriented programming techniques, including multiple inheritance, function overloading and template (generic) programming, which allow existing objects to be (re-)used in many different ways without having to change their original implementation.