In its present incarnation, OSCAR is a fully implemented programmable architecture for a rational agent. This architecture is described in detail in a forthcoming book entitled Cognitive Carpentry. It is an important characteristic of OSCAR that most practical reasoning is reduced to epistemic reasoning, through a process I call doxastification. In particular, most (but not quite all) of the details of planning, plan adoption, and plan execution are carried out by epistemic reasoning. If we focus just upon the epistemic reasoning in OSCAR, what we have is a powerful general-purpose defeasible reasoner. The purpose of this paper is to describe that reasoner.