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