In this work we study, theoretically, the magnetic properties of transition metals (TMs)-doped SnO2 (with TM = V, Cr, and Mn) in a diluted magnetic oxide configuration, focusing in particular in the role played by the presence of O vacancies, V-O, nearby the TM. We present the results of first-principles electronic structure calculations of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(V-O)(0.02) alloys. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a high-spin (HS) ground state, respectively, with 2 and 3 mu(B)/cell, for Cr and Mn, and a metastable low-spin (LS) state, with 0 (Cr) and 1 (Mn) mu(B)/cell. For vanadium, only a state with 1 mu(B)/cell was found. The spin-crossover energy (E-SCO) from the LS to the HS is 114 meV for Cr and 42 meV for Mn. By creating O vacancies close to the TM site, we show that the metastability and E-SCO change. For chromium, a new HS state appears (4 mu(B)/cell), with an energy barrier of 32 meV relative to the 2 mu(B)/cell state. For manganese, the metastable LS state of 1 mu(B)/cell disappears, while for vanadium the HS state of 1 mu(B)/cell remains. In all cases, the ground state corresponds to the expected HS. These findings suggest that these materials may be used in applications that require different magnetization states. (C) 2012 Elsevier B. V. All rights reserved.