You push a skateboard really hard with your foot. The skateboard O pushes back on your foot but not as hard and that's why there is a non-zero net force that accelerates it away. O pushes back on your foot as hard and that's why it would remain at rest due to the cancellation of the two forces. O doesn't push back on your foot at all and that's why it accelerates away due to your push. O pushes back as hard on your foot