“O.W.L.s are really important, affect the jobs you can apply for and everything.” -- Ron Weasley (OP12)
At the end of fifth year, each student sits an Ordinary Wizarding Level (O.W.L.) for each of the classes he or she takes. These are standardized tests administered by the Wizarding Examinations Authority; the teachers may proctor exams outside their own subjects but do not attend the Ordinary Wizarding Level (O.W.L.) in their own subject
Each O.W.L. has a theory portion, and for applicable classes a separate practical portion is given, so that many O.W.L.s are in two parts, although only one O.W.L. score is given for each subject. See the individual classes for the details of the material covered in the individual exams.
A note on what is disallowed during the O.W.L. examinations:
“Now, I must warn you that the most stringent Anti-Cheating Charms have been applied to your examination papers. Auto-Answer Quills are banned from the examination hall, as are Remembralls, Detachable Cribbing Cuffs, and Self-Correcting Ink. Every year, I am afraid to say, seems to harbor at least one student who thinks that he or she can get around the Wizarding Examinations Authority’s rules.” -- Minerva McGonagall (OP31)
From the Web
The maximum number of O.W.L.s by sushimustwrite