The magnetic field at the centre of a circular arc of radius $R$ carrying current $I$ is $B=\dfrac{\mu_0 I}{4R}$ (for a semicircle, using $\theta=\pi$ in $B=\dfrac{\mu_0 I\theta}{4\pi R}$). The straight radial segments produce no field at $O$ since current there flows along the line joining the point to $O$ ($d\vec{l}\parallel \hat{r}$, so $d\vec{l}\times\hat{r}=0$). The two arcs carry current in opposite senses relative to the centre, so their contributions are added or subtracted depending on direction.