The angular frequency of oscillation for a particle executing angular oscillations  with couple per unit twist of c is 

For a body executing simple harmonic motion, Restoring torque acting on it after a  small displacement  $\theta$, about an axis,
$\tau =-c\theta$  
We know that this results in angular oscillations which can be also related by the  equation $\tau =I\alpha$ 
Equating the above two we get  $-c\theta =I\alpha$
Where for angular oscillations  $\alpha =-{\omega }^2\theta$
Hence  $-c\theta =-I{\omega }^2\theta$
Simplifying this  $c=I{\omega }^2$
Hence  $\omega =\sqrt{\frac{c}{I}}$