We have investigated the structure and magnetism of atomic oxygen adsorbed Scn (n = 2–14) clusters by using the ab initio density functional theory approach. The oxygen atom tends to attack the hollow site in the ground state structures, and the bridge site in some metastable structures. The adsorption energies exhibit clear size-dependent variation, with maxima at n = 6, 10, 12 and minima at n = 4, 7 and 13, which can be assigned to the stability of the corresponding pure Scn clusters. The incoming O atom exhibits great influence on the magnetism of the clusters. The magnetic moments of the Scn clusters are partly or totally quenched at n = 2, 4, 6, 10, 12, 13, and 14, and enhanced at n = 7, 9, and 11 upon the addition of O, particularly the magnetic moment increases from 1 to 5 μB for n = 11 and decreases from 19 to 7 μB for n = 13. The different magnetic responses to O adsorption are further investigated using the Hirshfeld population analysis and partial and local density of states analysis.