Difference between Periodic Motion and Oscillatory Motion
Any motion that repeats itself at regular interval of time is called periodic motion. Examples: Motion of the pendulum of a clock, motion of planets around the Sun, etc.
A to and fro or back and forth motion of a body along the same path, without any change in shape of the body, is called an oscillatory motion. Examples: Motion of a swing or pendulum, motion of the needle of a sewing machine moving up and down, etc.
All oscillatory motions are periodic but NOT all periodic motions are oscillatory. For example, motion of a planet around the Sun is periodic but not oscillatory.