In this paper, the peak-to-average power ratio (PAPR) reduction problem with tone injection (TI) in the direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system is investigated, which is formulated as a tough integer combinatorial optimization problem. Since it is a challenging task to find the global optimal solution, the branch-and-bound method (BBM), which is extensively employed to deal with the NP-hard problem, is introduced to solve this problem. By splitting the superior branches and pruning the inferior branches, the close optimal solution is obtained. Simulation results reveal that the proposed BBM-based TI method has superior PAPR reduction and bit error rate (BER) performance compared with some existing algorithms. The proposed algorithm ensures a relatively low peak value, and thus provides an important benchmark in performance evaluations relative to other existing algorithms targeting at the same problem.