  This paper describes a new $\nu_e$ identification method specifically
designed to improve the low-energy ($< 30\,\mathrm{GeV}$) $\nu_e$
identification efficiency attained by enlarging the emulsion film scanning
volume with the next generation emulsion readout system. A relative increase of
25-70% in the $\nu_e$ low-energy region is expected, leading to improvements in
the OPERA sensitivity to neutrino oscillations in the framework of the 3 + 1
model. The method is applied to a subset of data where the detection efficiency
increase is expected to be more relevant, and one additional $\nu_e$ candidate
is found. The analysis combined with the $\nu_\tau$ appearance results improves
the upper limit on $\sin^2 2\theta_{\mu e}$ to 0.016 at 90% C.L. in the
MiniBooNE allowed region $\Delta m^2_{41} \sim 0.3\,\mathrm{eV}^2$.
