| Rating | Book | Author |
 (1) | Oaks of California | Bruce M. Pavlik |
| (1) | Odd Lots: Seasonal Notes of a City Gardener | Thomas C. Cooper |
| Of Flowers and a Village: An Entertainment for Flower Lovers | Wilfrid Blunt |
| (1) | Oklahoma Gardener's Guide | Steve Dobbs |
| (2) | Old Herbaceous: A Novel of the Garden | Reginald Arkell |
| Old Roses (Little Plant Library) | Andrew Mikolajski |
| (1) | Old Shrub Roses (revised ed.) | Graham Stuart Thomas |
| (1) | Old Time Herbs for Northern Gardens | Minnie Watson Kamm |
| On the Wild Side: Experiments in the New Naturalism | Keith Wiley |
| On-Farm Composting Handbook | Robert Rynk |
| Once and Future Gardener: Garden Writing from the Golden Age of Magazines 1900-1940 | Virginia Tuttle Clayton |
| Once upon a Windowsill: A History of Indoor Plants | Tovah Martin |
| One Hundred Desert Wildflowers of the Southwest | Janice E. Bowers |
| One Hundred English Gardens: The Best of the English Heritage Parks and Gardens Register | Patrick Taylor |
| One Hundred Great Garden Plants | William H. Frederick, Jr. |
| (1) | One Man's Garden | Henry Mitchell |
| (1) | Onward and Upward in the Garden | Katherine S. White |
| Orchid Basics: A Step-by-Step Guide to Growing and General Care | Isobyl LaCroix |
| Orchid Biology: Reviews and Perspectives | Joseph Arditti |
| Orchid Grower's Companion: Cultivation, Propagation, and Varieties | David Banks |
| Orchid Species Culture: Dendrobium (Orchid Species Culture) | Charles O. Baker |
| (1) | Orchids | Paul Starosta |
| Orchids | Greg Allikas |
| Orchids | I.D. James |
| Orchids (Practical Gardening Handbook) | Wilma Rittershausen |
| Orchids (Time-Life Encyclopedia of Gardening series) | James Underwood Crockett |
| Orchids and Their Conservation | Harold Koopowitz |
| Orchids for Dummies | Steven Frowine |
| Orchids for Every Home: The Beginner's Guide to Growing Beautiful, Easy-Care Orchids | Wilma Rittershausen |
| Orchids of Asia | Teoh E. Soon |
| (1) | Orchids of Britain & Europe | Pierre Delforge |
| Orchids of Europe, North Africa and the Middle East | Pierre Delforge |
| Orchids of Kenya | Joyce Stewart |
| (1) | Orchids of Papua New Guinea | Andrée Millar |
| Orchids Simplified | Henry Jaworski |
| Orchids to Know and Grow | Thomas J. Sheehan |
| Orchids, A Care Manual | Brian Rittershausen |
| Orchids: A Kew Gardening Guide | Joyce Stewart |
| (1) | Orchids: A Splendid Obsession | Wilma Rittershausen |
| Organic Gardening for Dummies | Ann Whitman |
| (1) | Organic Gardening for the 21st Century | John Fedor |
| Organic Gardening in Cold Climates | Sandra Perrin |
| Organic Vegetable Production: Proceedings from the New York State Agricultural Experiment Station in January 2003 | Abby Seaman |
| (1) | Oriental Vegetables | Joy Larkcom |
| Ornamental Foliage Plants | Denise Greig |
| (1) | Ornamental Grasses for Cold Climates | M. Hockenberry Meyer |
| Ornamental Grasses of the Southeast | Peter Loewer |
| Ornamental Herb Garden: Creating Compact Gardens | Catherine Mason |
| (2) | Ornamental Palm Horticulture | Timothy K. Broschat |
| Ornamental Shrubs, Climbers and Bamboos | Graham Stuart Thomas |
| (1) | Ornamental Tropical Shrubs | Amanda Jarrett |
| Ortho's All About Annuals (Ortho's All About Gardening) | Ann Lovejoy |
| Ortho's All About Attracting Birds (Ortho's All About Gardening) | Michael D. McKinley |
| (1) | Ortho's All About Attracting Hummingbirds and Butterflies | Ortho Books |
| Ortho's All About Azaleas, Camellias, and Rhododendrons (Ortho's All About Gardening) | H. Edward Reiley |
| Ortho's All About Bonsai | Penelope O'Sullivan |
| Ortho's All About Building Waterfalls, Ponds, and Streams | Charles M. Thomas |
| Ortho's All About Bulbs (Ortho's All About Gardening) | Marty Ross |
| Ortho's All About Citrus and Subtropical Fruits | Maggie Blyth Klein |
| Ortho's All About Container Gardening | Sally Roth |
| Ortho's All About Dry Climate Gardening | Denny Schrock |
| Ortho's All About Flowering Trees & Shrubs | L. Flint Harrison |
| Ortho's All About Garden Pools and Fountains (Ortho's All About Gardening) | Veronica Lorson Fowler |
| Ortho's All About Gardening: Creating Japanese Gardens | Alvin Horton |
| Ortho's All About Greenhouses | Ortho Books |
| Ortho's All About Ground Covers (Ortho's All About Gardening) | Katie Lamar Smith |
| Ortho's All About Herbs (Ortho's All About Gardening) | Maggie Oster |
| (1) | Ortho's All About Houseplants (Ortho's All About Gardening) | Kate Jerome |
| Ortho's All About Landscaping (Ortho's All About Gardening) | Jo Kellum |
| Ortho's All About Lawns (Ortho's All About Gardening) | Marilyn Rogers |
| Ortho's All About Orchids | Marilyn Rogers |
| (1) | Ortho's All About Perennials (Ortho's All About Gardening) | Ann Lovejoy |
| (1) | Ortho's All About Pruning (Ortho's All About Gardening) | Judy Lowe |
| (1) | Ortho's All About Roses (Ortho's All About Gardening) | Thomas Cairns |
| (1) | Ortho's All About Shade Gardening | Jo Kellum |
| Ortho's All About Shrubs and Hedges (Ortho's All About Gardening) | Penelope O'Sullivan |
| Ortho's All About Sprinklers and Drip Systems (Ortho's All About Gardening) | Ortho Books |
| (1) | Ortho's All About the Easiest Flowers to Grow (Ortho's All About Gardening) | Penelope O'Sullivan |
| Ortho's All About the Easiest Roses to Grow | Thomas Cairns |
| (1) | Ortho's All About Trees (Ortho's All About Gardening) | Jan Johnson |
| Ortho's All About Vegetables (Ortho's All About Gardening) | Barbara Pleasant |
| Ortho's All About Vines and Climbers (Ortho's All About Gardening) | R. William Thomas |
| Ortho's Guide to Successful Houseplants | Larry Hodgson |
| (1) | Ortho's Home Gardener's Problem Solver | Cheryl Smith |
| (2) | Otherwise Normal People: Inside the Thorny World of Competitive Rose Gardening | Aurelia C. Scott |
| (1) | Our Endangered Planet: SOIL | Suzanne Winckler |
| (1) | Our Stolen Future: Are We Threatening Our Fertility, Intelligence, and Survival?-A Scientific Detective Story | Theo Colborn |
| (1) | Out in the Garden: Growing a Beautiful Life | Dean Riddle |
| Outdoor Plants for Indoor Rooms | Kathy Sheldon |
| Outdoor Stonework: 16 Easy-to-Build Projects For Your Yard and Garden | Alan Bridgewater |
| Outdoor Water Features: 16 Easy-to-Build Projects For Your Yard and Garden | Alan Bridgewater |
| Over the Fence with Joe Gardener | Joe Lamp'l |
