oblong - a leaf that is much longer than wide.
obovate - a leaf the shape of an egg standing upside down with the narrow side at the bottom.
offset - a short, horizontal branch that grows from a plant's crown and bears buds and leafy rosettes.
olericulture - a branch of horticulture that deals with the science of the cultivation of vegetables.
one-year whip - refers to a one-year old un-branched tree with roots attached. This is a common classification for fruit trees which are purchased bare-root.
open-pollinated - plants that are pollinated by the wind or by an insect such as a bee without any manipulation by humans is said to be open-pollinated. For example, when a person goes into the garden and randomly collects seeds from a hosta plant in the fall, the resulting seedling is said to be open-pollinated.
opposite - the leaf and stem arrangement in which buds, leaves or stems are aligned directly opposite each other. Examples would include Maple (Acer), Honeysuckle (Lonicera), Deutzia, Viburnum.

See alternate and whorled.
 
organic - pertaining to living organisms in general, to compounds formed by living organisms and to the chemistry of compounds containing carbon.

In gardening terms, organic refers to any material that was once alive, or that comes from a living creature. Sawdust, compost, bone meal, guano are organic, while perlite, vermiculite or ammonium sulfate are inorganic.

organic gardening - There are many variations on this theme and there is no individual, all-encompassing definition. However, for most backyard gardeners, the term refers to gardening without synthetic (man-made) chemical pesticides or fertilizers.
organic matter - the decomposing bodies or parts of dead plants and animals. Organic matter contributes to the overall health of garden soils by providing nutrients to plants, holding moisture and nutrients and encouraging micro and macro organisms in the soil.
organic soil - soil composed of a high percentage of decayed plant and animal remains as opposed to the percent of mineral content. They often are associated with lake bottoms that have filled in with aquatic plant remains over thousands of years and are now considered peat sources. These are sometimes called muck soils. See mineral soil.
ornamental plant - a plant grown primarily for its esthetic traits rather than as a food source. A few plants, however, can fulfill both roles in the landscape.
ornamental horticulture - the branch of horticulture that deals with the cultivation of plants for their aesthetic value. It includes floriculture, landscape horticulture and, in some cases, turfgrass.
 
ovary - the site at the base of the female organ (pistil) where the eggs are produced. Once fertilized with pollen, the ovary contains the ovules which will develop into the seed(s) inside the fruit.
ovate - a leaf the form of an egg
overwinter - having the ability to survive the sub-freezing temperatures of a typical winter.

Also the process of helping the plant survive the winter such as temporary placement in a greenhouse or other structure, covering with extra mulch or, in the case of hybrid tea roses, covering with soil, styrofoam tubes or leaves.
ovules - the structure within the ovary that, after fertilization, develop into seeds.
own-rooted - used to differentiate between plants grown on their own roots versus those who have been grafted onto the root of another plant.

For example, many shrub roses are grown on their own roots which are perfectly hardy in cold climates. All hybrid tea roses are grafted onto a different rootstock because the hybrid rose plant roots tend to be non-hardy.
oxidation - the loss of an electron by an atom or molecule in the presence of oxygen.
 

 

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