Oliver returns to redo his NEWTS at the request of Dumbledore so that they have an Order member close enough to be able to protect Harry when professors are not near. He only accepts this mission on the condition that Fred and George were allowed join the order, attend meetings and assist him with this task. Much to Molly's displeasure which lead to Fred and George's delight this condition was accepted by Dumbledore as Albus thought three people protecting Harry was better than one person.