The given signal is \( v_s = 0.4 \sin(100\pi t) \). The diode has a forward voltage drop of 0.6 V. The OpAmp in the circuit will produce an output \( v_{OA} \) which follows the input signal, but it will be limited by the diode's voltage drop. Since the peak of \( v_s \) is 0.4 V, the diode will not conduct as the signal is lower than the diode's threshold voltage. Therefore, the output voltage \( v_{OA} \) will be limited to the peak value of the input signal minus the diode drop, which gives:
\[
v_{OA} = 0.4 - 0.6 = -0.2 \, \text{V}
\]
Thus, the value of \( v_{OA} \) is \( -0.2 \, \text{V} \).