Oak
The oak tree is an important source of wood for barrels. Although other woods, such as cherry, have been (and still are used), oak is the number one choice for wine barrels. See my article on wine and oak for more information.

Oaky
The number one choice of wood for wine barrels. It imparts toasty, vanilla, and smoky aromas to the wine.

Oechsle (Germany)
A scale of must weight based on specific gravity. It is vital to assess the must weight in Germany as it determines the Prädikat classification. An Oechsle unit is equal to one unit of specific gravity above 1000. For example, the must of a Mosel with specific gravity of 1090 has an Oechsle of 90 and therefore qualifies as an Auslese. Other measures of must weight include Baumé, Brix and KMW. My article on must weight in my Sweet Wine series gives more information.

Oenology
The science behind winemaking.

Off
A general word for any wine that is not quite as it should be. The wine could be faulty.

Off-dry
Not quite dry, with a touch of sweetness.

Oidium
A fungal disease, also known as powdery mildew. Like many vine diseases it thrives in damp conditions. It results in a reduction in quality and yields, but fortunately may be controlled with the application of sulphur, a practice permitted for even organic and biodynamic winemakers.

Old World Wine
Wine from European nations, such as France, Germany, Italy, Spain, Austria and others with a long history of viticulture.

Open
This describes a wine that's ready to drink, often because it’s been decanted (a process of opening up the wine by exposing it to oxygen). I also think of many people as open, especially around 5 pm when they’re ready to drink.

Organic Viticulture
The approach some winemakers take when they rely less or not at all on synthetic fertilizers, pesticides, and other chemicals. Those that meet certain criteria may be labeled as organic.

Oxidation
The distructive action of oxygen on a wine. The degrading action of oxygen on a wine (or any other substance) is known as oxidation. Hence exposure of the wine to oxygen in the winery is carefully controlled, although not necessarily completely avoided. Exposure to oxygen during racking and ageing in barrel can be of benefit to the wine. Once a bottle of wine has been opened for some time, or if oxygen has seeped past a faulty cork, the oxidised wine will taste off.