The porous nature of an oak barrel allows some levels of evaporation and oxygenation to occur in wine but typically not at levels that would cause oxidation or spoilage of the wine. In a year, the typical 59 gallon barrel can lose anywhere from 5 1/2 to 6 1/2 gallons of wine through the course of evaporation. This evaporation (of mostly alcohol and water) allows the wine to concentrate its flavor and aroma compounds. Small amounts of oxygen are allowed to pass through the oak barrel and acts as a softening agent upon the tannins of the wine.

The chemical properties of oak itself can have a profound effect on the wine. Phenols within the wood interact with the wine to produce vanilla type flavors and can give the impression of tea notes or sweetness. The degree of "toast" on the oak barrel can also impart different properties affecting the tannin levels of the wine as well as the aggressive wood flavors. The hydrolysable tannins present in wood, known as "ellagitannins", are derived from lignin structures in the wood. They help protect the wine from oxidation and reduction.

Wines can be barrel fermented in oak or they can be placed in oak after fermentation for a period of aging or maturation. Wine that is matured in oak receive more of the oak flavors and properties then wine that is fermented in oak. This is due to the fact that yeast cells interact with and "latch on" to the oak components. When the dead yeast cells are removed from the wine as lees some of these oak properties go with them. A characteristic of white wines that are fermented in oak include a pale color with an extra silky texture. White wines that are fermented in steel and then matured in oak will have a darker coloring due to the heavy phenolic compounds that are still present. Flavor notes that are common descriptions of wines exposed to oak include caramel, cream, smoke, spice and vanilla. Chardonnay is a variety that has very distinct flavor profiles when fermented in oak that include coconut, cinnamon and cloves notes. The "toastiness" of the barrel can bring out varying degrees of mocha and toffee notes in red wine.

The length of time that a wine spends in the barrel is dependent on the varietal and style of wine that the winemaker wishes to make. The majority of oak flavoring is imparted in the first few months that the wine is in contact with oak but a longer term exposure can effect the wine through the light aeration that the barrel allows which helps to precipitate the phenolic compounds and quickens the aging process of the wine. New World Pinot noir may spend less then a year in oak. Premium Cabernet Sauvignon maybe spend two years. The very tannic Nebbiolo grape may spend four or more years in oak. High end Rioja producers will sometimes age their wines up to ten years in American oak to get a desired earthy, vanilla character.

Studies have found that there maybe some health benefit from wine that has been in contact with oak. In 2003, scientists at Institut Européen de Chimie et Biologie in Pessac, France found that when the oak tannin vescalagin interact with phenols in wine a polyphenol known as acutissimin A is created which has been shown to be 250 times more effective then the pharmaceutical drug Etoposide in stopping the growth of cancerous tumors.