During most of its history, these forms were usually used with names of whole numbers only--"three o'clock," not "three thirty o'clock." Indication of day or night was done by specifying "six o'clock at night." The style you cite--"6:30 p.m. o'clock"--is a very formal style found in invitations and other rigid contexts. In most cases one would simply say "6:30 p.m.," but in formal situations the expression of time is felt to be incomplete without the "p.m." and the "o'clock." While this is indeed awkward, it is rare; it can be pretentious when used in non-formal ways. And very formal invitations have enough awkward elements to them that we probably shouldn't worry too much about how the time is stated.
From here.