When I came to "O" I immediately thought 'orchids' but then not everyone grows orchids and my tender ministrations did nothing for those I had and they all eventually went to orchid heaven. My son-on-law, on the other hand, has a couple of dozen cymbidium orchids in pots and they flower prolifically every year. I am always rather envious when I visit them and see the beautiful display.
The only other "O" that comes to mind is more a shrub than a bedding plant and yet I think they are very beautiful:
OLEANDER is an evergreen shrub in the dogbane family Apocynaceae, toxic in all its parts. It is the only species currently classified in the genus Nerium. It is most commonly known as oleander, from its superficial resemblance to the unrelated olive Olea. It is so widely cultivated that no precise region of origin has been identified, though southwest Asia has been suggested. The ancient city of Volubilis in Morocco may have taken its name from the Berber name oualilt for the flower. Oleander is one of the most poisonous of commonly grown garden plants.
Our neighbour has a very large shrub that protrudes into our back garden over the adjoining fence but it is high enough not to cause a problem to anyone. You often see them planted on road verges and in parks as well and I am always a little fearful that young children may want to pick the pretty flowers. Our two little great-grandies love to pluck a daisy or similar from our garden then they visit and lots of small children do love flowers and you do have to admit the flowers are glorious.