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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 predictedforeground contribution. Only these uncontaminated $k$-modes are used formeasuring $P(k_{\perp},k_{\parallel})$ in the "Foreground Avoidance" technique.Though the foregrounds are largely localised within a wedge. we find that thesmall leakage beyond the wedge surpasses the 21-cm signal across a significantpart of the $(k_{\perp},k_{\parallel})$ plane. The extent of foreground leakageis extremely sensitive to the frequency window function used to estimate$P(k_{\perp},k_{\parallel})$. It is possible to reduce the leakage by makingthe window function narrower, however this comes at the expense of losing alarger fraction of the 21-cm signal. It is necessary to balance these competingeffects to identify an optimal window function. Considering a broad class ofcosine window functions, we identify a six term window function as optimal for21-cm power spectrum estimation with OWFA. Considering only the $k$-modes wherethe expected 21-cm power spectrum exceeds the predicted foregrounds by a factorof $100$ or larger, a $5\,\sigma$ detection of the binned power spectrum ispossible 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 ofobservation respectively.

Simulated Predictions for HI at $z = 3.35$ with the Ooty Wide Field Array (OWFA) -- II: Foreground Avoidance