referring to different uses of ashwood as horsewhips, wood used by carpenters, and for spears. In the Old Irish period, onn "ash" was replaced by uinnius. McManus takes this as an indication that the Ogham letter names date to the Primitive Irish period. Primitive Irish is the oldest known form of the Irish language, known only from fragments, mostly personal names, inscribed on stone in the ogham alphabet in Ireland and western Britain up to about the 4th century. ...
References
Damian McManus, Irish letter-names and their kennings, Ériu 39 (1988), 127-168.
The origin of ogham is uncertain; it contained 25 letters formed of straight lines and may have been adapted from a sign language.
For this reason, the letter b is said to be named after the birch, and In Lebor Ogaim goes on to tell the tradition that all letters were named after trees, a claim also referred to by the Auraicept as an alternative to the naming after Fenius' disciples.
A letter for p is conspicuously absent, since the phoneme was lost in Proto-Celtic, and the gap was not filled in Q-Celtic, and no sign was needed before loanwords from Latin containing p appeared in Irish (e.g.
Onn is dedicated to the work of clearing away that which is not wanted - like the broom, but much more vigorous.
Onn calls for the "strength of warriors," but is also the "gentlest of work." For the work of Onn is never destruction for its own sake.
Onn represents the strength to be ruthless if necessary, to free yourself from destructive relationships, actions, or attitudes, no matter how deeply rooted they have become.