Ogam is an ancient script used by the Celts of the Brittish Isles, similar to the runes of the ancient Germanic peoples. Ogam inscriptions dating from approximately the fourth century of the common era have been found carved on standing stones in England, Wales, Scotland, and Ireland. It is believed that the chief use of Ogam was for magical and divinatory purposes, employed primarily by Celtic bards, seers and Druids to cast lots and record teachings, spells and incantations. Druidic Ogam inscriptions have been found to employ an archaic form of Celtic that would not have been in common use at the time the inscriptions were made. Some scholars have speculated that the Druids deliberately used archaic language and the Ogam script as a kind of "secret" language, to prevent unworthy commoners or foreign invaders from penetrating the occult secrets of the priesthood.

In the religion of the Druids, trees were sacred, and each character of the Ogam alphabet was associated with a type of tree. Different trees were associated with different spiritual qualities and powers. Below is a chart of the Ogam alphabet showing each Ogam's name, phonetic correspondence, the tree it was associated with, and its divinatory reference.

ogam	name	phonetic correspondence	tree	divinatory reference
	BEITH	B	BIRCH	faded trunk and fair hair
	LUIS	L	ROWAN	delight of eye
	FEARN	F	ALDER	shield of warriors
	SAILLE	S	WILLOW	hue of the lifeless
	NUIN	N	ASH	checking of peace
	HUATH	H	HAWTHORN	pack of wolves
	DUIR	D	OAK	highest of bushes
	TINNE	T	HOLLY	threefold
	COLL	C	HAZEL	fairest of trees
	QUERT	Q	APPLE	shelter of a hind
	MUIN	M	BLACKBERRY	strongest of efforts
	GORT	G	IVY	sweeter than grasses
	NGETAL	NG	FERN	a physician's strength
	STRAIF	ST	BLACKTHORN	strength of red
	RUIS	R	ELDER	most intense of blushes
	AILM	A	SCOTS PINE	loudest of groanings
	ONN	O	GORSE	helper of horses
	UR	U	HEATHER	in cold dwellings
	EDADH	E	WHITE POPLAR	distinguished wood
	IDHO	I	YEW	oldest of woods
	EABHADH	EA	ASPEN	most buoyant of woods
	OIR	OI	SPINDLE	most venerable of structures
	UINLLEAN	UI	HONEYSUCKLE	juicy wood is the woodbine
	IFIN	IO	GOOSEBERRY	sweestest of woods
	PHAGOS	AI	BEECH	expression of a weary one