This document defines the OTX core requirements and data model specifications.The requirements are derived from the use cases described in ISO 13209-1. They are listed in the requirements section.The data model specification aims at an exhaustive definition of all OTX core features implemented to satisfy the core requirements. Since OTX is designed for describing test sequences, which themselves represent a kind of program, the core data model follows the basic concepts common to most programming languages.Thus, this document establishes rules for syntactical entities like parameterised procedures, constant and variable declarations, data types, basic arithmetic, logic and string operations, flow control statements like loop, branch or return, simple statements like assignment or procedure call as well as exception handling mechanisms. Each of these syntactical entities is accompanied by semantic rules which determine how OTX documents are interpreted. The syntax rules are provided by UML class diagrams and XML schemas, whereas the semantics are given by UML activity diagrams and prose definitions.With respect to documentation use cases, special attention is paid to defining a specification/realisation concept (which allows for “hybrid” test sequences: human readable test sequences that are at the same time machine-readable) and so-called floating comments (which can refer to more than one node of the sequence).The core data model does not define any statements, expressions or data types that are dependent on a specific area of application.For the convenience of the user, the ISO 13209-2 OTX XML schema definition file (XSD) is published alongside this document.