| Cultivar Name | Plant Size | Origin | Flower Color | Season | Source | Registered |
| 'Oakview' | Small | Unknown | Med-Lavender | June into July | Stone, David & Ruh, Peter | .1987 |
| 'Oasis' (Ward) | Large | Unknown | Near-White | June into July | Ward, Dr Dick & Ward, Jane | .2012 |
| 'Oberland Blue Dimension' | Medium | Hybrid | - | - | Rank, Stefan | --NR-- |
| 'Ocean Tide' | Large | Hybrid | Near-White | June into July | Mondt, Manfred | --NR-- |
| 'Oceans Away' | Giant | Unknown | - | June into July | Hanson, Curt | --NR-- |
| 'Oceans Blue' | Large | Unknown | - | July into August | Schmidt, Bevie | .2009 |
| 'Ocean's Fury' | Large | Unknown | - | - | Rawson, Don | --NR-- |
| 'October Sky' | Medium | Hybrid | Near-White | August to September | Dean, Don | .2008 |
| 'Odd Ball' | Medium | Hybrid | Near-White | July | Benedict, Dr Ralph (Herb) | .1995 |
| 'Oder' | Medium | Unknown | Med-Lavender | July into August | Klose, Heinz | --NR-- |
| 'Odina' | Large | Unknown | - | - | Wade, Van | --NR-- |
| 'Oglala' | Giant | Unknown | - | - | Wade, Van | --NR-- |
| 'Oh! Susanna' | Medium | Hybrid | Med-Lavender | June into July | Reuter, Laurence & Gilligan, Madelon | .2015 |
| 'Ohh! Blue Beacon' | Small | Unknown | Near-White | June into July | Schulz, Ed | .2013 |
| 'Ohh! Blue Bounty' | Large | Unknown | Near-White | June into July | Schulz, Ed | .2011 |
| 'Ohh! Blue Bruiser' | Giant | Unknown | Med-Lavender | - | Schulz, Ed | .2011 |
| 'Ohh! Blue Granite' | Medium | Unknown | Near-White | June into July | Schulz, Ed | .2018 |
| 'Ohh! Blue Otto' | Small | Unknown | - | - | Schulz, Ed | .2009 |
| 'Ohh! His Blue Eyes' | Large | Unknown | Med-Lavender | June into July | Schulz, Ed | .2017 |
| 'Ohh! Ultra Aqua' | Medium | Unknown | - | - | Schulz, Ed | --NR-- |
| 'Ohio Blue' | - | Unknown | - | - | Wade, Van | --NR-- |
| 'OHS Bryan's Blue' | Medium | Hybrid | - | - | Ontario Hosta Society | --NR-- |
| 'OHS Juno' | Medium | Sport | Pale-Lavender | July into August | Ontario Hosta Society | --NR-- |
| 'OHS Margot's Delight' | Medium | Sport | Pale-Lavender | July into August | Ontario Hosta Society | --NR-- |
| 'Ojibway' | Medium | Unknown | - | - | Wade, Van | --NR-- |
| 'Okobaji' | Small | Unknown | - | - | Tompkins, Chet & Ruh, Peter & Ruh, Jean | .2008 |
| 'Old Bill' | Giant | Hybrid | Pale-Lavender | June into July | Gilligan, Madelon | .2012 |
| 'Old Cadillac' | Large | Unknown | Pale-Lavender | June | Diesen, Bernie & Diesen, Jane | .1998 |
| 'Old Cardboard Boxes' | Large | Unknown | - | - | Marcelle, Eric | --NR-- |
| 'Old Frosty' | Large | Unknown | - | - | Rawson, Don | --NR-- |
| 'Old Gnarly' | Large | Hybrid | Pale-Lavender | - | Kruer, George | .2009 |
| 'Old Rough House' | Giant | Unknown | - | - | Wade, Van | --NR-- |
| 'Olivia' | Medium | Unknown | Near-White | - | Schwarz, Jim | --NR-- |
| 'Olympic Silver Medal' | Medium | Sport | Pale-Lavender | July into August | Fransen, Marco & Naylor Creek Nursery | --NR-- |
| 'Omaha' | Large | Unknown | - | - | Wade, Van | --NR-- |
| 'Oma's Sunrise' | Large | Unknown | - | - | Sisson, Kathie | --NR-- |
| 'Ontario Blue' | Large | Unknown | - | - | Wade, Van | --NR-- |
| 'Open Hearted' | Giant | Unknown | - | July | Wade, Van | .1999 |
| 'Orion's Belt' | Large | Hybrid | Pale-Lavender | July | Ward, Dr Dick & Ward, Jane | .2003 |
| 'Osiris Blue Night' | Large | Unknown | - | - | Fafard, Serge | --NR-- |
| 'Osiris Sentinelle' | Large | Unknown | - | - | Fafard, Serge | --NR-- |
| 'Osprey' | Medium | Unknown | Pure-White | June into July | Smith, Eric & British Hosta and Hemerocallis Society | .1988 |
| 'Our Friend Reldon' | Large | Hybrid | - | - | Van Wyke, Trudy | --NR-- |
| 'Ovalwood' | Large | Hybrid | Near-White | July | Beilstein, Doug & Beilstein, Mardy | .2012 |
| 'Over the Waves' | Medium | Unknown | - | August to September | Benedict, Dr Ralph (Herb) & Kuenster, Rod | .2010 |
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