Small amounts of oxygen in wines are absorbed by various Ions and consequently oxidizes them, you then end up with a domino effect, whereas other components are being oxidized by the oxidized Ions in the wine. A wine that receives excessive oxidation rather quickly will combined with other constuients such as ethanol, thus causing unwanted formation of aldehydes in the wine. A good example of this is bottle sickness in a wine after bottling. Fortunately if the level of oxygen in not excessive the wine overcomes this and begins to improve. However wines continuously exposed to excess oxygen over time, will begin to decline in both flavor and aroma.
Lets look at some ways to prevent excessive oxygen from entering wine during the winemaking process.
The most important aspect of keeping wines free of oxygen is to keep all your containers full. During primary fermentation, some airspace is acceptable. Your fermenting wine will contain lots of carbon dioxide during primary fermentation, thus preventing oxygen from entering the wine. Once primary is over though, keep your wine containers full at all times. See topping up page for tips on keeping containers full.