What it means
Oak is the tree most people picture when they think of strength, and the sign follows suit. Oak people are said to be dependable and calm under pressure, with a strong sense of duty toward the people in their care. They tend to stand up for others before themselves and stay the course long after others have given up. They respect tradition, like a settled base and grow slowly into positions of trust.
Strengths
Strength under pressure, protectiveness, loyalty, generosity toward people in trouble, a long attention span, and plain honesty.
Challenges
Stubbornness, reluctance to change course, carrying everyone's weight without asking for help, and a temper that is slow to start and slow to cool.
Oak dates and Ogham letter
Oak runs from June 10 to July 7 (28 days). Its Ogham letter is Duir, the letter for the sound D. The dates come from the tree calendar Robert Graves set out in The White Goddess (1948): thirteen 28-day months named after Ogham letters in his Beth-Luis-Nion order, plus one spare day, December 23. It is a modern system inspired by the Ogham, not a calendar recorded from the ancient Druids, and scholars think only some of the Ogham letter names first meant trees. On the Western calendar, Oak starts in Gemini and the Sun moves into Cancer around June 21, so roughly the first 11 days are Gemini Suns and the last 17 are Cancer.
The oak in folklore
The Oak month holds midsummer, and Graves built a story around that: the Oak King rules the growing half of the year until the solstice, when the Holly King takes over. The Irish word dair means oak, and a popular etymology links the word druid to it as 'oak knowledge', though linguists still argue about that one. Graves also tied Duir to the English 'door', reading the month as the gateway of the year.
How Cosmic Array reads it
Your tree depends only on the day and month you were born, so the year and birth time don't change it. The Architect blueprint shows your Celtic tree next to your Chinese animal, Mayan sign and Egyptian deity, and its written synthesis can set the tree against your Western Sun, Moon and Rising.