Actually, Publius Cornelius Tacitus was referring to the runes (or some precursor or derivative thereof) not the Ogham. The quote is from Tacitus' Germania. There is a version in the original latin
here, or in english
here. It's a relativly short read, & somewhat informative, however, remember that it was written by a Roman, so it is out of cultural context (Tacitus apparently did try very hard to be impartial, however). Linguistic evidence seems to support a date of around 200BCE, however they could come from a much earlier date. I remember seeing a newer discovery of the earliest set, but I can't recall the date. I'll have to look through my notes for it.