Optical networks transport an ever increasing volume of traffic, composed by signals with multiple modulation formats. In this scenario, in-band crosstalk due to imperfect isolation of optical devices can degrade significantly the network performance. In this work, an analytical formalism, based on the moment generating function, for evaluating the performance of differential phase-shift keying (DPSK) optical receivers impaired by crosstalk due to on-off keying (OOK) interferers is developed and validated by Monte Carlo simulation. In comparison with DPSK interferers, the crosstalk tolerance is decreased 1dB. It is also observed that the extinction ratio of the OOK interferers, practically, does not influence the receiver performance.