# Along which axis is the moment of inertia of a harmonically oscillating body calculated?

I have been learning about oscillating bodies and recently stumbled upon physical pendulums. Now the problem is i don't understand about which axis is the MOI calculated while finding the TIME PERIOD(T)

for example if i have a curved surface of radius=$$R$$ with a small ball of radius $$r$$ (please ignore the blue line ,the ball is not connected to any point and the ball is rolling without slipping) .

so my doubt here is should i calculate the MOI along the centre of mass or the edge of the ball /:)