Abrotanella emarginata (Gaudich.) Cass. (original description)
Lepidaploa (Cass.) Cass. (original description)
Oligactis (Kunth) Cass. (original description)
Oligactis apodocephala Cass. accepted as Oligactis sessiliflora (Kunth) H.Rob. & Brettell (original description)
Oligactis nubigena (Kunth) Cass. (original description)
Oligactis volubilis (Kunth) Cass. (original description)
Oligocarpha nereifolia (L.) Cass. accepted as Brachylaena neriifolia (L.) R.Br. (original description)
Oligosporus Cass. accepted as Artemisia L. (original description)
Oligosporus condimentarius Cass. (original description)
Onotrophe Cass. accepted as Cirsium Mill. (original description)
Onotrophe acaulis (L.) Cass. accepted as Cirsium acaulon (L.) Scop. (original description)
Onotrophe oleracea (L.) Cass. (original description)
Onotrophe palustris Cass. (original description)
Ormenis bicolor Cass. accepted as Cladanthus mixtus (L.) Chevall. (original description)
Orthocentron (Cass.) Cass. accepted as Cirsium Mill. (original description)
Orthocentron glomeratum Cass. (original description)