I think "o' clock" is only used by 12 hour time, not sure. It does make sense since a clock does not have more than 12 divisions, and the origin of the phrase obviously involves a clock :o
From what I understand, "o' clock" can be used with either time system, but "Hundred hours" is for military time, which uses the 24-hour system, but not all 24-hour systems are military time. (Run-on sentence?) Kind of like the square-rectangle relationship, or something.