OK+ means the reader discovers solid problems and articulates them well. This appears to depend on the quality of the code base. Because all code bases are somewhat flawed, however, the quality of the presented code doesn’t really affect your ability to get this grade.
OK means the reader asks pertinent questions and discovers some problems.
OK- means the reader asks questions and some of them are pertinent.
ZERO means the reader didn’t ask any questions or asked too few questions pertinent to the code/design at hand.