Question

all oscillatory motion are periodic motion explain

Answer 1

While oscillatory motion will almost certainly be periodic, not all periodic motion will be oscillatory. A common example is the orbit of the earth around the sun. A complete orbit takes roughly 365 days and therefore this is a periodic motion. 

Answer 2

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. On the other hand, all periodic motions may not be oscillatory.