Sirius’ OWLs were good, but his NEWTs were the worst, because during his school years, he never bothered to learn, but the Black family enforced him to do so and Arcturus threatened Sirius that he would be disinherited from the heirship, if he didn’t do well in school. So, Sirius didn’t have any choice then to work for his exams and his OWLs. He did indeed well on his OWLs due to the influence of his family, but he failed six NEWTs after he escaped to the Potters, which caused that many people were mocking him to be a failure of a Black. That had caused Sirius nearly to go on a killing spree. The man failed herbology, arithmancy, ancient runes, potions, history of magic and astronomy. This had led that he would never get his position as an auror back, if he ever wanted to return, but due to his mastery in DADA, which Albus encouraged him to do, if someday the tampering was discovered, Lucius wasn’t able to fire him. Sirius was ready to kill someone, when his NEWT results came to him and ultimately ended his career as an auror.