  We present the absolute properties of the eclipsing binary AO Ser with a
pulsating component from our $BV$ photometric and high-resolution spectroscopic
observations, which were performed between April and May 2017. The radial
velocities (RVs) for both components were measured, and the effective
temperature and projected rotational velocity of the primary star were
determined to be $T_{\rm eff,1}$ = 8,820 $\pm$ 62 K and $v_1 \sin i_{1}$ = 90
$\pm$ 18 km s$^{-1}$, respectively, by comparing the observed spectrum with the
Kurucz models. The accurate fundamental parameters of AO Ser were determined by
a simultaneous analysis of the light and RV curves. The masses and radii of the
primary and secondary components are $M_1$ = 2.55 $\pm$ 0.09 M$_\odot$ and
$R_1$ = 1.64 $\pm$ 0.02 R$_\odot$ and $M_2$ = 0.49 $\pm$ 0.02 M$_\odot$ and
$R_2$ = 1.38 $\pm$ 0.02 R$_\odot$, respectively. Multiple frequency analyses
for the eclipse-subtracted light residuals were conducted. As a result, we
detected two frequencies of $f_1$ = 21.852 days$^{-1}$ and $f_2$ = 23.484
days$^{-1}$. The evolutionary position on the HR diagram and the pulsational
characteristics indicate that the primary star is a $\delta$ Sct pulsator with
a radial fundamental mode. On the other hand, the relatively evolved secondary
is oversized for its own mass.
