OpenMark is an eAssessment system in its own right but also one that can supply questions to the Moodle eAssessment system (the Moodle Quiz) through the Opaque protocol (http://docs.moodle.org/24/en/Opaque_question_type.
At the OU we see them as complementary. Moodle has authoring facilities for standard questions but Moodle cannot go beyond its standard questions. In contrast OpenMark has no easy authoring tools beyond an editor such as Eclipse but authors have complete flexibility in building their questions from OpenMark components.
You may read more about the influence of OpenMark on Moodle here http://www.open.edu/openlearnworks/course/view.php?id=1581.