Using Ampere's law and the magnetic field equation for a straight conductor, we calculate the magnetic field induction at the point \( O \). The formula for the magnetic field due to a current in a straight conductor is \( B = \frac{\mu_0 I}{2 \pi r} \). Since the point is along the perpendicular bisector of the conductor, the induction at point \( O \) is half of that.
Step 2: Conclusion.
The magnetic field induction at point \( O \) is \( \frac{\mu_0 I}{2r} \), corresponding to option (a).