How is diffusion involved in osmosis?

Hint: Osmosis is a special kind of diffusion because the water molecules are moving from a region of high concentration to a low concentration through a semipermeable membrane. Diffusion doesn’t need a partially permeable membrane which osmosis usually does.

Complete answer:
Osmosis is a known specific type of diffusion; it is the passage of water from a region of high water concentration from a semi-permeable membrane to a region of low water concentration.
Semi-permeable membranes are usually very thin layers of material which allow some things to pass through them but prevent other things that are passing through. Cell membranes are some examples of semi-permeable membranes.
Cell membranes allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but usually do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.
The membrane allows the solvent (water) to move through but keeps out the solute (the particles which are dissolved in the water). Transport by osmosis is usually affected by the concentration of solute (the number of particles) in the water
But it always involves another molecule actually and that often is salt. A "law" of osmosis says that water always follows the salt where salt is higher in their concentration, water will be following

Note: Diffusion does not require any semipermeable membrane while osmosis does require it When osmosis is usually a unidirectional phenomenon; the diffusion occurs in all directions. Further, in osmosis, the only solvent which flows while in diffusion, all constituents move.