Oak:  steadfastness ...

Olive: peace ...

Oleander:  caution ...  

Orange Blossom:  innocence ... eternal love ...  marriage and fruitfulness ... happy marriage ...

Orchid, all:  love ... beauty ... refinement ... beautiful lady ... a belle ... lust ... luxury ...

Orchid, cattleya:  mature charms ...

Orchid, cymbidium:  magnificence ...

Orchid, phalaenopsis:  evening joy ...