Local density functional theory is used to investigate theinteraction of nitrogen with oxygen in Cz-Si. The binding energyof O with N2 and the positions of four local vibrational modesof the N2O center are in good agreement with experiment. TheN2O2 defect is the most common nitrogen-oxygen defect after N2Oand we suggest that the experimentally observed lines at 1018and 810 cm-1 are due to this defect. The concentrations of thesedefects are greater than those of oxygen dimers at temperatures around 650{degree sign}C. Furthermore thesedefects could be nuclei for oxygen precipitates in nitrogendoped Cz-Si.Neither NO nor NO2 play any significant role.