Drift velocities and reactions of negative ions have been investigated in oxygen at temperatures between 310 and 370 °K at values of EN (the ratio of electric field to neutral-gas number density) between 2 and 400 × 10-17 V cm2, and at pressures (P0) between 0.5 and 4 Torr. Electrons were produced from a hot filament and reacted with the neutral-gas molecules to produce ions. The ions were then gated into a drift cell through a double grid, withdrawn through an aperture, and their mass and intensity were determined as a function of drift time by a quadrupole mass analyzer and a gated particle multiplier. Comparison of experimental results with numerical calculations allowed the determination of ion drift times and reaction rates. The rate constant for the reaction O-+O2+O2-->O- 3+O2 was found to be 9 × 10-31 cm6/sec at low EN (<30×10-17 V cm2). At higher EN (>70×10-17 V cm2) the reaction O-+O2-->O-2+O was observed with a rate that increased sharply with EN to a value of 5 × 10-11 cm3/sec at EN=300×10-17 V cm2.