There are basic differences between osmosis and diffusion. Osmosis can be simply defined as the movement of water molecules from the region of lower concentration to the region of higher concentration through a semipermeable membrane. The water molecules from the lower concentration are moving to the region of higher concentration to equalize the concentration of both sides of the membrane. It requires a semipermeable membrane before it can be called osmosis.
In osmosis, only water molecules can pass through the semipermeable membrane While, diffusion can be defined as the movement of particles from the region of lower concentration to the region of higher concentration. The particle moving from one region to another can be anything including water and it does not necessarily have to pass through a semipermeable membrane. In diffusion, solvent and solute particles can move from one region of lower concentration to another region of higher concentration since it doesn't require a semipermeable membrane.