Opposite (leaved)
Used to describe a plant having a pair of leaves at a node on opposite sides of the stem.
Orbicular
Circular in shape, referring to leaves or sometimes flowers.
Organic gardening
The cultivation of plants without the use of chemicals of inorganic origin.
Ovate
A leaf that's shaped like an egg, broadly oval usually with a tapering point.
Ovate-acuminate
An egg-shaped leaf that culminates in a long point.
Ovate-cordate
An egg-shaped leaf with a heart-shaped base.
Ovoid
An egg-shaped leaf, usually without a tapering point.
Ovule
The organ containing the egg within the ovary of a flower. After fertilisation, it develops into a seed.
Oxygenator
An aquatic plant, which, when submerged, releases oxygen into the water.