The electric field at the center of a uniformly charged circular ring is zero because the fields produced by diametrically opposite elements cancel each other out. According to the principle of superposition, the net electric field at the center ($\vec{E}_{total}$) is the vector sum of the field due to arc AKB ($\vec{E}_{AKB}$) and the field due to the rest of the ring, arc ACDB ($\vec{E}_{ACDB}$).

$\vec{E}_{total} = \vec{E}_{AKB} + \vec{E}_{ACDB} = 0$

This implies: $\vec{E}_{ACDB} = -\vec{E}_{AKB}$

This means the field due to ACDB must be equal in magnitude to the field due to AKB ($E$) but opposite in direction. Since the charge is positive, the field $\vec{E}_{AKB}$ is directed away from the arc AKB, i.e., from K towards O (along KO). Therefore, the opposing field $\vec{E}_{ACDB}$ must be directed from O towards K (along OK).