>>16654210A green oak is a YOUNG oak.
Plants that haven't grown enough and unripe fruit are green, fresh wood is also called green wood.
People that aren't mature or old enough are often described as "green" for that reason.
Your rival, the Green Oak, is an inmature trainer.
His grandpa, the old Oak, is an experienced professor.
Can't believe I had to explain that, this term isn't even limited to the English language.
