We describe a theoretical treatment of the recombination spectrum of O II. The ab initio calculations are carried out in intermediate coupling which allows the distribution of population among the 3P$_J$ ground levels of O$^{2+}$ to be correctly incorporated for the first time. The effects of dielectronic recombination due to states lying between the 3P$_J$ levels is also included. The new theory allows the strongest O II recombination lines to be used as a diagnostic of the temperature and density of the emitting region and illustrative examples are given.