We use a global (volume averaged) model to study the presence of negative ions and metastable species in low-pressure high-density oxygen discharges. We find the negative oxygen ion O- to be the dominant negative ion in the discharge, the density of the negative ion O2- to be small and the density of the negative ion O3- to be negligible in the pressure range of interest, 1-100 mTorr. Dissociative attachment of the oxygen molecule in the ground-state O2(3Σg-) and the metastable oxygen molecule O2(a1Δg) are the dominating channels for the creation of the negative oxygen ion O-. At low pressure (<5 mTorr) recombination involving O- and O+ ions is the main loss channel for O- ions. At higher pressure, the detachment on O(3P) becomes the main loss channel for the O- ion. The creation of O2- is mainly through dissociative attachment of ozone O3. Ozone is almost entirely created through detachment by the collision of O- with the metastable oxygen molecule O2(a1Δg). The creation of O2- is thus greatly influenced by this detachment process and neglecting the detachment has a significant influence on the density of O2- ions. At low pressure (<10 mTorr) the O2- ion is mainly lost through recombination while at higher pressure the charge transfer to form O2 is the dominating loss process.