We propose differential overlapping pulse position modulation (DOPPM) in optical direct-detection channel to achieve higher capacity and cutoff rate than pulse position modulation (PPM), differential PPM (DPPM) or overlapping PPM (OPPM) when the bandwidth and the average power of the channel are constrained. In order to achieve higher throughput, the proposed modulation scheme allows more than one pulse-positions per pulsewidth over DPPM. In DOPPM the slots after the optical pulse in the block are deleted by using differential technique and the next symbol starts just after the optical pulse of the previous symbol. To avoid frame synchronization problems, we analyze the performance of DOPPM under the window scheme that causes the degradation on the performance of DOPPM but is easy to analyze. We analyze both the capacity and the cutoff rate of DOPPM under this scheme. It is shown that DOPPM has higher capacity and cutoff rate than PPM, DPPM and OPPM when the average received number of photons per slot is somewhat large.