Oak: A type of hardwood commonly used for building wine barrels. American oak has a distinctive, bourbon-like coconut flavour but French oak flavour is much more subtle. Both types of oak barrels contribute considerable tannin and vanilla flavours to wines during aging.

Oenology: ("ee-nol-o-gee," ignore the initial o altogether) Oenology, also spelled enology, comes from the Greek word oenos, which means 'wine'. Oenology is the study of wine and winemaking. This is thought of differently than viticulture, which is the study of grapes and grape growing. Someone experienced at winemaking is called an "oenologist" or "oenologist." Remember not to embarrass yourself by pronouncing the “o.” Oenology is pronounced ee-nol-ogy, period.

Oloroso: One of the categories of Spanish Sherry. Olorosos are "bigger" and fuller in body, flavour and sweetness than Fino Sherries.

Oporto: Largest seaport city in northern Portugal. This is the gateway to the port wine region.

Osmosis: The natural movement of fluids through a membrane or porous partition such as a cell wall. Fluid tends to move through the membrane towards a solution of higher concentration so as to equalize the concentrations on both sides of the membrane. That's important to a grower watching his vines grow or to a winemaker who wants to process a wine to remove excess volatile acidity, alcohol or other component by "reverse osmosis." This is one more example of how complex winemaking has become since the advent of technology.

Over cropped: A vine that carries more crop than it can reasonably ripen. Vines that aren't pruned drastically enough tend to set too much crop. Wine produced from fruit of an over cropped vine is always poorer in quality than if the crop were normal size. An over cropped vine can be corrected, if it's done in time, by simply thinning the crop in late June or early July. The grower sends in a crew to cut off from 10 to 40% of the over cropped fruit while it is small and green. The remaining fruit will then develop, ripen correctly and produce better wine than it would have if the thinning had not taken place.

Oxidation: The chemical reactions involved in combining oxygen with wine to produce "oxidized" changes in the flavours and colour of the wine. In table wines, oxidation is almost always undesirable, and irreversible. Once ruined, the wine stays ruined. Oxidation can be defined as any adverse change in wine flavour, stability and/or colour caused by excessive exposure to air.