Marking programming assignments involves a lot of work, and with large classes, the feedback provided to students through marking is often rather limited and late.Oto is a customizable and extensible marking tool that provides support for the submission and marking of assignments. Oto aims at reducing the marking workload and, also, at providing early feedback to students.In this paper, we present Oto's new marking language and give an overview of its implementation as a Domain-Specific Language.