Disrupting the CD40-CD40L costimulation pathway promotes allograft acceptance in many settings. Herein, we demonstrate that stimulating OX40 overrides cardiac allograft acceptance induced by disrupting CD40-CD40L interactions. This effect of OX40 stimulation was dependent on CD4(+) T cells, which in turn provided help for CD8(+) T cells and B cells. Allograft rejection was associated with donor-reactive Th1 and Th2 responses and an unconventional granulocytic infiltrate and thrombosis of the arteries. Interestingly, OX40 stimulation induced a donor-reactive IgG class switch in the absence of CD40-CD40L interactions, and the timing of OX40 stimulation relative to transplantation affected the isotype of donor-reactive Ab produced. Inductive OX40 stimulation induced acute graft rejection, which correlated with both IgG1 and IgG2a deposition within the graft. Once graft acceptance was established following CD40-CD40L blockade, delayed OX40 stimulation did not induce acute allograft rejection despite priming of graft-reactive Th1 and Th2. Rather, chronic rejection was induced, which was characterized by IgG1 but not IgG2a deposition within the graft. These studies reveal both redundancy and key differences in function among costimulatory molecules that manifest in distinct pathologies of allograft rejection. These findings may help guide development of therapeutics aimed at promoting graft acceptance in transplant recipients.