Oxygen in Winemaking
A highly reactive gas, oxygen is a threat to winemakers and contact with oxygen is limited. Cold grapes are less effected. Antioxidants like Sulfur Dioxide are used, grapes are picked at night (it's cooler), the grapes are chilled until they reach the winery, winery equipment is flushed of oxygen and airtight. Wine is stored in stainless steel or concrete lined with glass or epoxy. This new-world style is called Anaerobic Winemaking.
Oxygen in Maturation
Wines that are matured in oak have contact with small amounts of oxygen. Small vessels provide more contact, and wine in stainless tanks can be oxidized through bubbling. Oxidation helps develop complexity and depth in aromas, soften tannins, and makes the color stable. Too much oxygen destroys the fruit character of wine, makes it smell stale, and gives it a brown tinge.
S02 acts as an antioxidant and an antiseptic, added in powder form as grapes are picked and levels are adjusted throughout the winemaking process. High levels are toxic and can make the wine harsh / lack fruit.
Oak Winery Vessels
Oak vessels are used in fermentation and maturation of wine to add tannins, stabilize color and extract aroms of toast, vanilla and smoke. European Oak is more expensive than American Oak, due to wastful and labor-intensive production processes European is toast and vanilla, American is sweet vanilla and coconut.