An ab initio CASPT2//CASSCF study of the (3)A(') ground potential energy surface for the O(P-3)+N2O(X (1)Sigma (+)) reaction has been performed, investigating the two predominant reactive channels. Symmetry breaking is reported for some of the structures. Rate constants are calculated by means of the transition state theory yielding values in almost quantitative agreement with experiment for the 2 NO(X (2)Pi) channel, but at variance with experiment for the N-2(X (1)Sigma (+)(g))+O-2(X (3)Sigma (-)(g)) one. A preliminary study on the possible existence of surface crossings ((3)A(')-(1)A('), (3)A(")-(1)A("), and (3)A"-(3)A(') intersections) reveals that more efforts are warranted to fully explain the origin of this discrepancy.