You're about to take six subject-specific tests, one for each Hogwarts' core class, except History of Magic, 'cause where nicer than Dolores Umbridge, and an elective courses test. Due to the limitation of being muggles, you'll take the written parts only, judged on a scale of "troll" to "outstanding".
ASTRONOMY O.W.L.
CHARMS O.W.L.
DEFENCE AGAINST THE DARK ARTS O.W.L.
HERBOLOGY O.W.L.
POTIONS O.W.L.
TRANSIGURATION O.W.L.
THE END
http://harrypotter.wikia.com/wiki/Ordinary_Wizarding_Level