A category in a classification of globular clusters, in which they are divided into Group I or Group II according to their abundances of heavy metals. Group I clusters have higher metal abundances, somewhat brighter horizontal branch stars, and slightly shorter periods for their RR Lyrae variables (0.55 days compared with 0.65 days) than Group II clusters. Omega Centauri is an Oosterhoff Group II cluster, the iron abundance of its stars being only a few per cent of the Sun’s value. They are named after the Dutch astronomer Pieter Theodorus Oosterhoff (1904–78).