It depends what you mean. We could breathe with plants, but trees are the true supply. A fully grown oak tree provides 8 people with oxygen for a lifetime.
Most trees which are trees are trees, also when they're not. Sometimes a tree which is actually a tree but a tree afraid to admit he is a tree is called a busheee.
Trees "breathe" CO2 (carbon dioxide) through extremely small openings between the plants cells on the bottom of the leaf, called stomata (stoma singular). Through the same stomata O2 leaves the leaf.