The Ordinary Wizarding Level Grading system are as follows:
Exceeds Expectations (E)
Harry's grades were:
Care of Magical Creatures: E
Defense Against the Dark Arts: O
History of Magic: D
I think Harry could have done better in Astronomy and History of Magic, but during Astronomy McGonnagal was being hexed by Umbridge, so he wasn't very focused. And during History of Magic he passed out.