O
Name: Onn - ash tree; wheel, turning, movement, sole of the foot, foundation
Welsh: onen - ash tree, possibly cognate with L: ornus - mountain ash or rowan
Root: IE: *os-, *osen-, ash-tree; OI: onn, ash tree (though it is more often uinniuis or uinnsiu by the period when the Book of Ballymote was written); OI: (f)onnaid, turning, swift movement, wheel of a chariot, wanderer
Color: odhar - dun
Tree: furze, ash, whin, heather
Bird: odoroscrach - scrat
MM: congnaid ech: helper of horses i.e. the wheels of a chariot, wounder of horses; hastener of horses (horsewhip)
MO: féthem soíre: smoothest of work; smoothest of craftsmanship
CC: lúth fían; desire of the fianna i.e. heather, sustaining equipment of warrior or hunting bands
Deities: Manannán as Gatekeeper and Lord of Mists, Láeg the Charioteer of Cú Chulainn, Maíle Dúin

Meanings: travel, journeys, movement in a static situation, spiritual journey work, working within the Otherworlds, guidance, guiding spirits, building foundations
Notes: Two word ogams here point to the meaning of wheel or movement, as does the OI fonnaid. The idea of the horsewhip suggests furze, while the sustaining equipment of a warrior or hunter would suggest spears, made from ash wood.
My preference is for the wheel, turning, movement meaning set based on fonn, sole of the foot (and foundation) -- which implies walking or journeying -- or (f)onnaid suggested by Meroney. McManus doesn't even suggest this for a possible interpretation, preferring the "ash" reading.
Fonnaid has four distinct meanings: the wheel of a chariot, swift movement, a wanderer, and turning, movement or removal. The Duil Feda ("Elements of Letters") section in the Book of Ballymote uses the word "onnaid" to indicate the wheels of a chariot, and is a variant spelling of fonnaid. All of these meanings for fonnaid lead me to the idea of movement and journeys, and the other meanings expand from that basis.