The problem with Ogham is that even the Irish texts are not in agreement as to its origin and there is, to my knowledge at least, no unequivocal reference to Ogham in the Welsh materials. Furthermore, the archaeological and linguistic evidence (the correspondences between letters and sounds) points to its being developed within the context of Primitive Irish, around the 4th century CE, so basically on the eve of the Christian period in Ireland and the twilight of Roman Britain. In terms of the history and geographical spread of Celtic-speaking cultures across Europe, Ogham is relatively limited and relatively late. Ogham stones don’t show up in earlier Gaulish, Celtiberian or Galatian contexts – all contexts in which druids and parallels with Insular Celtic culture and myth do; moreover, they don’t even show up in Brittany either.
The Ogham script is commonly called the Tree Alphabet, but in actual fact there are numerous different forms of Ogham, including Saint Ogham, Bird Ogham, Sow Ogham and even River Ogham. The tree interpretations seem to have “taken over” Ogham to the exclusion of a lot of the other material – especially the kennings. Furthermore, not all of the names of the letters in the tree listing even refer to trees anyway, for example, the sixth letter – úath, adding even more confusion. Finally, the idea that Cad Goddeu is about Ogham is largely down to The White Goddess, by Robert Graves, which, despite Graves’s unquestionable poetic talent, is not a text that anyone seriously interested in Celtic culture, myth and legend should rely on at all.