  Considering the upcoming OWFA, we use simulations of the foregrounds and the
$z = 3.35$ HI 21-cm intensity mapping signal to identify the
$(k_{\perp},k_{\parallel})$ modes where the expected 21-cm power spectrum
$P(k_{\perp},k_{\parallel})$ is substantially larger than the predicted
foreground contribution. Only these uncontaminated $k$-modes are used for
measuring $P(k_{\perp},k_{\parallel})$ in the "Foreground Avoidance" technique.
Though the foregrounds are largely localised within a wedge. we find that the
small leakage beyond the wedge surpasses the 21-cm signal across a significant
part of the $(k_{\perp},k_{\parallel})$ plane. The extent of foreground leakage
is extremely sensitive to the frequency window function used to estimate
$P(k_{\perp},k_{\parallel})$. It is possible to reduce the leakage by making
the window function narrower, however this comes at the expense of losing a
larger fraction of the 21-cm signal. It is necessary to balance these competing
effects to identify an optimal window function. Considering a broad class of
cosine window functions, we identify a six term window function as optimal for
21-cm power spectrum estimation with OWFA. Considering only the $k$-modes where
the expected 21-cm power spectrum exceeds the predicted foregrounds by a factor
of $100$ or larger, a $5\,\sigma$ detection of the binned power spectrum is
possible in the $k$ ranges $0.18 \leq k \leq 0.3 \, {\rm Mpc}^{-1}$ and $0.18
\le k \le 0.8 \, {\rm Mpc}^{-1}$ with $1,000-2,000$ hours and $10^4$ hours of
observation respectively.
