Olea europaea 'Arbosana' makes for an excellent table olive or a rich, fruity, delectable oil with a lovely mild flavour.

Ophiopogon 'Black Beard' is taller growing than 'Nigrescens' to one foot high forming fuller, more vigorous clumps.

Opuntia aurea 'Coombes Winter Glow' is a hardy evergreen Prickly Pear with bright pink flowers and green pads that turn dark purple in winter.

Opuntia macrorhiza (Charles B.) has rich, tangerine orange flowers.

Opuntia polyacantha 'Crystal Tide' has beautiful oval to diamond shaped, olive green pads with golden and white spines and silver pink flowers.

Opuntia "Pink and Blue" has bluish pads that flush purple in winter and pink flowers.

Opuntia 'Red Gem' has small, round, 2-3 inch wide pads with short spines and deep pink to magenta flowers.

Origanum 'Gilt Trip' is an ornamental oregano with lilac flowers, mauve calyxes and aromatic, chartreuse-yellow foliage.

Origanum 'Bellissimo' is an ornamental oregano with pendulous purple-pink and white hop-like flower heads and green hop-like foliage.

Origanum 'Drops of Jupiter' is an ornamental oregano with pink flowers and aromatic, chartreuse-yellow foliage.

Ornithogalum reverchonii is a rare star-of-Bethlehem with spikes of fragrant, outward-facing, pure white, bell-shaped flowers on stems 24-36 inches tall.

Osmanthus fragrans f. aurantiacus has small, orange, extremely fragrant, edible flowers famous in China for use in teas and other foods.

Osmanthus fragrans f. thunbergii has small, pale yellow, extremely fragrant, edible flowers famous in China for use in teas and other foods.

Osmunda cinnamomea, the cinnamon fern, has lush green vegetative fronds and cool rusty orange sporophytic fronds.

The green, shamrock-like leaves of Oxalis violacea give rise to lavender or rose-pink flowers in spring.