Elasto-inertial microfluidics enables precise particle focusing and separation, but most experimental studies rely on fluorescence microscopy, which provides only 2D information and cannot resolve out-of-plane motion. Here, we combine fluorescence imaging with Optical Coherence Tomography (OCT) to obtain complementary lateral and depth-resolved (resolution along the beam: 2.58 μm in the present medium) particle distributions in microfluidic channels. Using 3 μm and 5 μm particles in a PEO solution at flow rates of 1–50 μL/min, fluorescence microscope captures lateral focusing followed by lateral defocusing with the increasing flow rate, while OCT reveals the vertical distribution of particles at the focusing and defocusing states. These results demonstrate that OCT is a promising method to obtain essential 3D information in elasto-inertial flows that complements fluorescence microscopy and enables a more complete understanding of particle behavior in elasto-inertial flows.