The line-of-sight (LOS) requirement of free-space optical (FSO) systems can be relaxed by employing optical intelligent reflecting surfaces (IRSs). In this paper, we show that an IRS-assisted FSO system employing a square-law photo detector (PD) receiver can be modeled as a linear end-to-end system if the receiver lens area is sufficiently large. Based on this linear model, we characterize the impact of IRS-induced delay dispersion and derive an analytical expression for the corresponding channel impulse response (CIR), which reveals the dependence of the end-to-end channel on the characteristics of the incident and reflected beams’ wavefronts, the position of transmitter and receiver, the size and phase shift profile of the IRS, and the incident beamwidth on the IRS. For transmission, we consider an on-off keying (OOK) and a DC-clipped optical orthogonal frequency-division multiplexing (DCO-OFDM) system. Our simulation results reveal that a maximum effective delay spread of 0.7 ns is expected for in-plane reflection from a square IRS with an area of 1 m2, which induces ISI for bit rates larger than 10 Gbps. We show that while the maximum delay spread is approximately independent of the IRS phase shift profile, the received power for focusing and quadratic phase shift profiles is larger than that for linear phase shift profiles. We also show that the IRS-induced delay dispersion can be mitigated by equalization at the receiver. Our results reveal that DCO-OFDM performs better than OOK modulation with zero forcing linear equalization (ZF-LE), whereas OOK modulation with decision feedback equalization (DFE) always outperforms DCO-OFDM.