In Ogmund's Gift, we learn that he was an enthusiastic member of the Up and Down Morris and Sword Dancers.
During The Dark Portal, he was most often seen with Piccadilly, the cheeky city mouse. They went in search of Audrey Brown's mousebrass in the sewers. Oswald's main part in the book is when he disguised himself as a rat to regain the mousebrass. However, he was found out by Finn, a one-eared rat and almost killed. Oswald managed to kill Finn by pushing him onto his own knife. He was knocked into the sewer water when he confronts Jupiter.
He played a minor role in The Crystal Prison, where he was dying of an illness he caught shortly after leaving the sewers. However, he was revived by his cousin, William Scuttle and his good friend Audrey Brown with the Starwife's medicine.
His importance in The Final Reckoning rivaled that of Audrey herself. He and Arthur were the ones to discover that the food is gone. Sometime after that the Starwife lit a beacon fire to summon the bats, Orfeo and Eldritch. Oswald's sensitive eyes spotted the bats in the distance, and he was borne off to the Council of Bats, and placed before the Bat Elders. He collected the Book of Hrethel and bombarded Jupiter with the paper, wounding him terribly. However, Oswald didn't survive the adventure. The Green Mouse gave his parents the mousebrass he would have received if he had come of age - the Sign of Bravery.