Final answer:
Students with below an Outstanding grade in their O.W.L.s could not enter Snape's N.E.W.T. classes.
Step-by-step explanation:
Students with below an Outstanding in their O.W.L.s were not allowed to enter Snape's N.E.W.T. classes. Only students who achieved an Outstanding on their O.W.L.s were eligible for Snape's advanced classes. The grading system at Hogwarts consisted of grades from T, which is Troll, the lowest, to O, which is Outstanding, the highest. Therefore, students who did not achieve an Outstanding grade in their O.W.L.s would not be able to continue in Snape's N.E.W.T. classes.