Ogham is on much firmer ground than most divinatory alphabets, because it is attested twice over and in two quite different ways. Keeping the two apart is the whole discipline of reading it honestly.
The stones
Around four hundred ogham inscriptions survive, the great majority in Ireland, Kerry, Cork and Waterford above all, with a few dozen more in Wales, Scotland, the Isle of Man and the south-west of England. They are cut as strokes across or either side of an arris, the edge of an upright stone, and read from the bottom up.
They date to roughly the fourth to sixth centuries, and their content is monotonous in a way that is itself informative. They are almost entirely personal names, generally in a formula of the type X son of Y or X descendant of Y, marking a grave or a boundary or a claim.
Not one of them is an oracle, a spell, or a divinatory text. Nobody has ever found ogham used for anything but writing names on stone.
The manuscripts
The second body of evidence is later, larger and much stranger. Medieval Irish scholars wrote about ogham at length, and the principal collection is In Lebor Ogaim, the Ogham Tract, transmitted alongside the grammatical treatise Auraicept na n-Eces and preserved most fully in the fourteenth-century Book of Ballymote.
These texts supply things the stones do not:
- The letter names (beithe, luis, fern, sail, nion and the rest), and the division into groups called aicmi.
- The briatharogaim, three lists of two- and three-word kennings glossing each letter, ascribed to Morann mac Main, Mac ind Oc and Cu Chulainn. These are the earliest interpretations of the letters that exist.
- Something like a hundred alternative oghams, elaborate substitution schemes assigning the letters to birds, rivers, forts, colours, occupations, saints and much else.
Why the alternative oghams matter
Because they show what the medieval scholars were doing. This was grammatical and scholastic play, virtuoso systematising by literate men who found an alphabet and built taxonomies on it. Bird-ogham is not a bird oracle; it is an exercise.
The tree correspondences belong to that same body of scholastic material. They are genuinely medieval, and they are genuinely a later interpretive layer laid over letter names that had come from somewhere else.
That is the honest position: the letters are ancient, the letter names are early, the systematic tree scheme is medieval learned elaboration, and the divinatory use is modern.