Osmosis is the movement of solvent through a semipermiable membrane from a less concentrated solution to a more concentrated one.
The pressure generated by osmosis is described by the van 't Hoff equation:
$$\Pi V = nRT$$
Where n is the number of particles, V is the volume, R is the ideal gas constant, T is the absolute temperature. This is because solute particles in dilute solution behave like an ideal gas.
Colloid osmotic pressure is the osmotic pressure generated by plasma proteins, which retains water in the vascular space. Because the capillary is partly permeable to albumin, the capillary oncotic pressure is given by
$$\sigma [\pi_p - \pi_i]$$
where \(\sigma\) is the reflection coefficient (1 for a perfect semipermiable membrane, 0 for a membrane permiable to solutes). \(\sigma\) is \(\approx 1\) in glomeruli and \(\approx 0\) in liver sinusoids.