Onoclea - Hardy Fern - deciduous hardy perennial fern that prefers sheltered damp area. Beneath a garden wll on North side seems to suit.
Ophiopogon - Ophiopogon - Ornamental Grass - Black Grass - but also golden and silver variegated types. Shares the common name of Lilyturf, with the Liriope plant. Other than its 'chat about' features, I have never seen the black grass put to any substantial ornamental use, but it is different!
Origanum - Some very good foliage types have evolved recently for the Origanum group of perennials. Most of them are evergreen and not too tall so well suited to front of border. Origanum are of course herbs also.
Osteospermum - Osteospermum - Cape Daisy The Cape Daisies are well suited to growing on dry banks where they do a good groundcover job. that is not to dismiss the brightly coloured flowers. It also does well at the seaside. Keep deadheading the flowers if you want continuation.
Oenothera - The Evening Primrose - First hand experience of growing these in all sorts of places, including dappled shade. Have to say that full sun in a dry place bought the best results. Atop a dry stone wall being surprising. The traditional golden yellow is also augmented by some pink shades.
Olsynium - member of the Iris family and sharing the names of the Sisyrinchium group of plants to confuse things more. Often wrongly named, so we will do what we can to put that right.
Omphalodes - Navelwort is common name for a reason unknown to me!). Member of the Borage family and shares many of its basic characteristics. mainly blue flowers on upright stems, held above rich green foliage. but, there are also some white ones.
Ononis - Restharrow - A short lived perennial member of the Pea family. Prefers sun and is not always hardy as is sometimes suggested. But well worth growing, though you might think of it as more a sub-shrub.
Onopordum - The Cotton thistle - More a biennial than perennial, but does self-seed. Very imposing.
Orchis - Hardy Orchids - Are you one of the lucky ones to have a situation where you can grow hardy orchids? They will need a well drained semi-shaded area - and lots of luck!
Ornithogalum - Hardy Bulb - Star of Bethlehem - White, bright and star-like, so a good choice of common name. Most are hardy and low growing. Some are upright - watch this space.
Osmunda - Hardy Fern - Unlike ferns generally, one of these - Osmunda regalis - has a flower - hence 'The Flowering Fern'. The foliage is also a little different to other ferns - but a fern it is.
Ostrowskia - A large flowered member of the Campanula family. Has been mistaken as being a lily because of the flowering size of Ostrowskia magnifica!
Othonna - Yellow Aster-type flowers on a low growing clump of silver grey leaves. Not fully hardy unless given winter protection.
Ourisia - Variable group of low growing evergreen perennials, normally forming a dense mat. Good on the rockery or in a specialist trough.
Oxalis - Shamrock - There are good as well as
bad Oxalis. It's a matter of choice and not simply buying a pot of
flowers - which can be so appealing at the garden centre!