The attachment coefficient η for the formation of negative ions in low energy electron swarms was measured over an E/p-range of 0.1 ... 30 V/Torr cm by a new and accurate method. Thus earlier measurements up to 54 Torr of other authors could be extended to 880 Torr. The shape of the minimum in the η/p-curves between the three-body process (e+2O2 → O2- + O2) and the dissociative process (e + O2 → O- + O) and its shift to higher E/p with increasing pressure was measured. Behind the minimum a maximum at E/p = 14 was found. Between the minimum and this maximum the dissociative process is predominant but the three-body process is still of influence. For E/p > 14 the η/p-values are slowly decreasing with increasing E/p. For the higher pressures above 44 Torr deviations from the relation η proportional to p2 were found for the three-body process. These deviations are discussed.