How does osmosis occur?
Osmosis is the diffusion of water where molecules of water move passively (without energy) from high concentration to low concentration.
A good example would be the way water rushes into a sponge, when a dry sponge is placed in a pan of water. Water fills the empty spaces of the sponge until the concentration in and out of the sponge is normalized.
Here is a video which describes how osmosis impacts onion cells placed into tap water and then into a salt water solution.
Hope this helps!