If $v$ is the speed of solid sphere when it reaches at point $O$, then by the law of conservation of energy,
$m g h=\frac{1}{2} I \omega^{2}+\frac{1}{2} m v^{2}$
$=\frac{1}{2} \cdot \frac{2}{5} m R^{2}\left(\frac{v}{R}\right)^{2}+\frac{1}{2} m v^{2}$
$\Rightarrow m g h=\frac{1}{5} m v^{2}+\frac{1}{2} m v^{2}$
$\Rightarrow g h=\frac{v^{2}}{5}+\frac{v^{2}}{2}$
$\Rightarrow g h=\frac{7 v^{2}}{10}$
$\Rightarrow v^{2}=\frac{10 g h}{7}$
$\Rightarrow v=\sqrt{\frac{10 g h}{7}}$
