The costimulatory molecule OX40 (CD134) is required in many instances for effective T cell-mediated immunity, controlling proliferation, and survival of T cells after encountering specific Ag. We previously found that the functional targets of OX40 are survivin and aurora B that regulate proliferation and Bcl-2 antiapoptotic family members that regulate survival. However, the intracellular pathways from OX40 that mediate these effects are unclear. In this study, we show that OX40 signaling can target the canonical NF-κB (NF-κB1) pathway in peripheral Ag-responding CD4 T cells. Phosphorylation of IκBα, nuclear translocation of NF-κB1/p50 and RelA, and NF-κB1 activity, are impaired in OX40-deficient T cells. Retroviral transduction of active IκB kinase that constitutively activates NF-κB1 rescues the poor expansion and survival of OX40-deficient T cells, directly correlating with increased expression and activity of survivin, aurora B, and Bcl-2 family members. Moreover, active IκB kinase expression alone is sufficient to restore the defective expansion and survival of OX40-deficient T cells in vivo when responding to Ag. Thus, OX40 signals regulate T cell number and viability through the NF-κB1 pathway that controls expression and activity of intracellular targets for proliferation and survival.