All oscillatory motions are periodic because each oscillations gets completed in a definite interval of time. Eg: On being pulled and then released, a load attached to a spring executes oscillatory motion.

Are all periodic motion oscillatory by nature?

All oscillatory motions are necessarily periodic motion but all periodic motion are not oscillatory.

Which is not oscillatory motion?

Solution: Oscillatory motion is the to and fro motion of a particle which is repeated over a fixed interval of time. Clock does not shows oscillatory motion.It shows periodic motion not oscillatory motion. Clock does not shows to and fro position about a mean position.