A teratoma of the ovary composed exclusively or predominantly of a single type of tissue derived from the ectoderm or endoderm. A representative example is struma ovarii which is a teratoma composed exclusively or predominantly of thyroid tissue. The disease is ovarian monodermal and highly specialized teratoma (Monarch Disease Ontology entry MONDO_0002372). Also known as: ovarian germ cell monodermal and highly specialised teratoma, ovarian germ cell monodermal and highly specialized teratoma.