This study investigated the effects of octenyl succinic anhydride-modified starch (OSA starch) at different concentrations (0, 0.5, 1.0, 1.5, and 2.0 g/100 mL) on the emulsion properties and volatile flavor compounds of snakehead fish soup. The results showed that the oil droplets in the fish soup became smaller and more uniformly distributed as the addition level of OSA starch increased. Confocal laser scanning microscopy (CLSM) revealed that the average diameter of oil droplets decreased from 5.98 μm in the control group to 4.24 μm at an OSA starch concentration of 2.0 g/100 mL. The addition of OSA starch reduced the adsorption of proteins at the oil-water interface, leading to a gradual decrease in the absolute value of the emulsion’s zeta potential, while the contact angle of the emulsion increased. Electronic nose (E-nose) analysis indicated that the response values of sensors W1C and W2S, which are sensitive to aromatic compounds and alcohols, respectively, increased with higher OSA starch concentration, peaking at 1.0 g/100 mL before declining. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that the relative contents of volatile compounds such as nonanal, (E)-2-heptenal, and trans-2-octenal were significantly higher in the OSA starch-treated groups than those in the control group, while the content of 1-octen-3-ol generally elevated. In conclusion, increasing the OSA starch concentration progressively reduced the oil droplet size in the fish soup emulsion, thereby promoting the release of volatile flavor compounds.