Or

Or stands apart from every other ogham letter, forfeda included, because its gloss is not a plant. The Auraicept na n-Eces names it for or, gold, itself a loanword into Irish from the Latin aurum. Every other letter in the twenty original aicmi and every other forfeda takes its manuscript gloss from a tree or shrub, which is why the alphabet is popularly known as a tree alphabet at all. Or breaks that pattern outright, and the tract tradition does not pretend otherwise.

The letter was devised to carry a diphthong close to oi, a sound the original twenty characters, built for an earlier stage of the language, had no single sign for. Its function is the same as the other forfeda: patching a phonetic gap that opened as Irish changed between the period the core alphabet was cut and the period scholars were writing grammatical tracts about it. What makes Or worth noting on its own is what its gloss reveals about how that patching was done, by reaching for whatever word fit the sound, tree or not, rather than preserving a naming convention some later readers assume was inviolable. The Latin loan aurum itself descends from an older Indo-European root for a yellowish or reddish colour, so even a supposedly non botanical gloss carries its own layered borrowing history behind it.

Sources Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.) with A Guide to OgamDamian McManus

The tree naming convention that gives ogham its popular identity holds for the original twenty letters with reasonable consistency, though even there scholars debate how much of it is a genuine mnemonic tradition and how much is a later gloss imposed by the grammarians who wrote it down. Or is useful precisely because it shows the convention breaking in real time. A scribe reaching for a word to carry a specific sound picked or, gold, a borrowed word with no botanical content, over inventing or straining toward a tree term that did not fit.

That single exception argues against reading the forfeda as tree letters in the same sense as the stone bound twenty. It argues for reading them as what the textual evidence actually shows: a scribal solution to a spelling gap, glossed after the fact with whatever word suited the sound, sometimes a tree and sometimes not. The four other forfeda do carry tree or shrub glosses, which keeps the appearance of the older convention intact for a reader moving quickly through the list. Or is the letter that stops a careful reader from assuming the appearance is the whole story, and a useful check against any modern chart that lists all twenty five ogham letters as trees without qualification.

Sources A Guide to OgamDamian McManus with Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.)