Which is best?
I think the poster meant...
(connotes too much a "poster", i.e. a piece of paper on the wall)
I think the author meant...
(the person who posted hasn't written enough to be considered an "author")
I think the original poster meant...
(sounds odd, why "original"?)
I think the OP meant...
(I am not sure people know what it stands for, I think it stands for "original poster" but am not even sure, and it connotes an operating room in a hospital.)
I think the person who posted meant...
seems the best but if you repeat it too often, your message becomes unnecessarily wordy