The cross section and average γ-ray multiplicity 〈M〉 were measured for reactions of O16 + Sm154 leading to specific xn and αxn final channels at five O16 energies from 111 to 168 MeV. For the 6n, 7n, and 8n channels the cross sections show nonstatistical tails and 〈M〉 saturates at high energies, indicating that emission of one or more fast neutrons is occurring. The 〈M〉 data suggest that the partial waves involved in this nonstatistical behavior are predominantly peripheral. Consideration of all available 〈M〉 data from this and other systems indicates that a strong correlation exists between the structure of the projectile and the first appearance of nonstatistical emission of neutrons and alpha particles. The generalized critical angular momentum model of incomplete fusion is extended, providing successful predictions for all data of the energy at which nonstatistical neutron emission sets in and of the energy variation of 〈M〉 in the various xn exit channels. NUCLEAR REACTIONS Sm154(O16,xnγ)Yb170−x, x=5−11, Sm154(O16,αxnγ)Er166−x, x=4−10, E=111.1, 126.4, 141.1, 155.4, 168.4 MeV; measured σ(E), average and standard deviation of γ-ray multiplicity distributions, average γ-ray energies; deduced onset of nonstatistical neutron emission, systematics of γ-ray multiplicity, incomplete fusion.