| Cultivar Name | Plant Size | Leaf Color | Variegation | Var Color | Origin | Flower Color | Season | Source | Registered |
| 'Oasis' (Ward) | Large | Blue/Green | - | - | Unknown | Near-White | June into July | Ward, Dr Dick & Ward, Jane | .2012 |
| 'Occupy Wall Street' | Small | Blue/Green | Marginal | White | Unknown | - | July into August | Freeman, Franc | .2012 |
| 'Oder' | Medium | Blue/Green | - | - | Unknown | Med-Lavender | July into August | Klose, Heinz | --NR-- |
| 'Ohh! Lovilia' | Large | Yellow | - | - | Unknown | Near-White | June into July | Schulz, Ed | .2015 |
| 'Ohio River' | Medium | Med-Green | - | - | Hybrid | - | - | Beilstein, Doug & Malloy, Alex | --NR-- |
| 'Okefenokee' | Medium | Med-Green | Marginal | Yellow | Sport | Pale-Lavender | July into August | Benedict, Dr Ralph (Herb) & Walek, Kevin | .2009 |
| 'OKUHIDA' | - | - | - | - | Unknown | - | - | Unknown | --NR-- |
| 'Old Faithful' | Large | Med-Green | - | - | Hybrid | Pure-White | August to October | Brincka, Wm & Petryszyn, 0lga | .1991 |
| 'Old Jerusalem Road' | Large | Blue/Green | Streaked | White | Unknown | - | - | Goodenough, Rick | --NR-- |
| 'Old Wethersfield' | Large | Dark-Green | | | Unknown | Light-Purple | | Goodenough, Rick | --NR-- |
| 'Olympic Glacier' | Medium | Dark-Green | Marginal | Yellow | Sport | Med-Lavender | July into August | Naylor Creek Nursery | .2005 |
| 'Omaha' | Large | Blue/Green | - | - | Unknown | - | - | Wade, Van | --NR-- |
| 'Ontario Blue' | Large | Blue/Green | - | - | Unknown | - | - | Wade, Van | --NR-- |
| 'Ooltewah' | - | - | - | - | Unknown | - | - | Unknown | --NR-- |
| 'Ophir' | Medium | Light-Green | - | - | Unknown | Pale-Lavender | - | Fischer, Dr Ulrich | --NR-- |
| 'Orinoco Flow' | Medium | Med-Green | - | - | Hybrid | Pale-Lavender | July into August | Cross, Peter | .2002 |
| 'Oshkosh' | - | Med-Green | - | - | Hybrid | - | - | Fisher, Eunice V. | --NR-- |
| 'Otara's Angel' | Large | Dark-Green | Marginal | Yellow | Sport | Near-White | June into July | Johns, Eddie | --NR-- |
| 'Otara's Sunrise' | Large | Med-Green | Marginal | White | Unknown | - | - | Johns, Eddie | --NR-- |
| 'Ottawa' | Large | Med-Green | - | - | Unknown | - | - | Wade, Van | --NR-- |
| 'Otter Point' | Large | Med-Green | - | - | Unknown | Med-Lavender | - | Wrede, Arthur | --NR-- |
| 'Outer Banks' | - | Blue/Green | Streaked | White | Hybrid | - | - | Goodenough, Rick | --NR-- |
| 'Oxford Blue' | Large | Dark-Green | Medial | Yellow | Hybrid | - | - | Cullerton, Walter | --NR-- |
| 'Oxford Blue Top' | Miniature | Blue/Green | | - | Unknown | Light-Purple | August to September | Colley, June & Baker, John | .2020 |
| 'Ozark Welcome' | Medium | Light-Green | Medial | Yellow | Sport | Pale-Lavender | June into July | Helsley, Calvin | .2004 |
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