Malolactic fermentation (MLF) is the bacterially driven decarboxylation of L-malic acid to L-lactic acid and carbon dioxide, and brings about deacidification, flavour modification and microbial stability of wine. The main objective of MLF is to decrease wine sourness by a small increase in wine pH via the metabolism of L-malic acid. Oenococcus oeni is the main lactic acid bacterium to conduct MLF in virtually all red wine and an increasing number of white and sparkling wine bases. Over the last decade, it is becoming increasingly recognized that O. oeni exhibits a diverse array of secondary metabolic activities during MLF which can modify the sensory properties of wine. These secondary activities include the metabolism of organic acids, carbohydrates, polysaccharides and amino acids, and numerous enzymes such as glycosidases, esterases and proteases, which generate volatile compounds well above their odour detection threshold. Phenotypic variation between O. oeni strains is central for producing different wine styles. Recent studies using array-based comparative genome hybridization and genome sequencing of three O. oeni strains have revealed the large genomic diversity within this species. This review will explore the links between O. oeni metabolism, genomic diversity and wine sensory attributes.