The fourth ogham family cuts short strokes square across the line, and Onn takes two of them.
How the letter is formed
Onn is the second letter of the fourth family, Aicme Ailme, and is cut as two short strokes crossing the line at a right angle. On stone these are frequently notches on the arris that do not extend onto the faces at all.
Notches are the least durable characters
Because the fourth family is cut short and directly on the edge, it is the first family to disappear as a stone weathers. The arris is the most exposed part of the surface, and a notch has less material behind it than a stroke that runs down a face. Inscriptions that survive with their consonants legible and their vowels uncertain are a recognised pattern in the corpus, and readings of such stones are correspondingly cautious about vowel counts.
The sound
Onn carries the value O. The surviving orthodox inscriptions, a few hundred stones in Ireland and in Irish-settled parts of Wales, Scotland and the Isle of Man, are almost entirely personal names in the genitive case, dated on linguistic grounds to roughly the fourth to the sixth centuries.
The later layers
The gorse attached to this letter comes from the medieval manuscript glosses, and the calendar and divinatory schemes in modern circulation descend from twentieth-century work later still. The atlas records the three as three, rather than presenting the modern material as though it came off the stones.