AO Serpentis, an eclipsing binary system, was studied using new multiband B, V, R light curve solutions and period variation analysis. Binary Maker 3 and Wilson–Devinney modeled light shapes. A semi-detached binary had a mass ratio q=0.22, inclination i=87.6°, and component effective temperatures T1=9480 K and T2=4786 With primary and secondary masses M1=1.56 M☉ and M2=0.62 M☉, and radii R1=1.67±0.02 R☉ and R2=1.47±0.03 RM3≈0.30 M likely caused long-term decline (dP/dt= -5.39(±0.03)×10−7 days yr−1) and cyclic oscillation (P3=17.3 year) across 80 minimum light periods. BVR light curves reveal a slight O'Connell effect, but the second maximum is ~0.03 mag fainter, indicating a secondary cold spot. Optical magnitudes (MV1≈+2.3 and MV2≈+5.1), bolometric corrections (BC1=−0.14, BC2=−0.46), and logarithmic ages (log τ ≈ 9.1) support late main-sequence.

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