The Object Oriented Finite-Element project at NIST <http://www.ctcms.nist.gov/oof> is a Python and C++ tool designed to bring sophisticated numerical modeling capabilities to users in the field of Materials Science. The software provides numerous tools for constructing finite-element meshes from microstructural images, and for implementing material properties from a very broad class which includes elasticity, chemical and thermal diffusion, and electrostatics. The current series of releases has a robust interface for defining new nonlinear properties, and provides both first and second order time-dependence in the equations of motion. The development team is currently working on a fully-3D version of the code, as well as expanding the scope of available properties to include surface interactions, such as surface tension and chemical reactions, and inequality constraints, such as arise in mechanical surface contact and plasticity. The software is a hybrid of Python and C++ code, with the high level user interface and control code in Python, and the heavy numeric work being done in C++. The software can be operated either as an interactive, GUI- driven application, as a scripted command-line tool, or as a supporting library, providing useful access to users of varying levels of expertise. At every level, the user-interface objects are intended to be familiar to the materials-science user. This presentation will focus on an interesting example of a nonlinear property, called Ramberg-Osgood elasticity, and the process for incorporating this feature into the OOF architecture.