Oncotic pressure is a type of osmostic pressure caused by non-easily solvated molecules (a.k.a protiens). Since the proteins cannot cross the capillary walls, they act similarly to any solute in a solution in which a semi-permeable blocks their movement down their concentration gradient. Imagine a salty water solution on one side of a U-tube, a pure water solution on the other... separated by a semipermeable membrane.
Because the Na+ / Cl- cannot cross the membrane, water wants to travel down its concentration gradient towards the salty solution to help... the side with salt gains pure water. Imagine the water it gains stays completely pure and gets placed nicely on TOP of the salty solution rather than mixing in. The pressure that column of pure water applies is enough to cancel the pulling power of the salt concentration gradient because for every water molecule pulled in, the extra hydrostatic pressure of the column pushes one out (rho g h). That pulling power of the salt solution could be negated by this water displacement or by pumping a higher atmospheric pressure on the air on top of the salty solution... water is not necessary. So like MedPR was saying, osmotic pressure often leads to hydrostatic pressure to reach equilibrium BUT an external atmospheric pressure could accomplish the same thing. Thus, Osmotic pressure and hydrostatic look like they go hand in hand but it's only because of your exposure to them.
Note that the column of water pulled in by osmotic pressure does not get placed nicely on top of the salty solution... it mixes in. BUT the two ways of looking at it are equivalent physically.
The pressure that negates this pulling power (the pure column of water on top of the salty solution) is hydrostatic pressure and in the context of physio is ONLY the external pressure exerted by the heart. If the blood pressure increases, you essentially increased the external pressure, artificially adding to the column of fluid on top of the salty solution. In reality you are increasing the volume of the blood... Unlike in the U tube however the body can sustain a non-equilibrium state in which it puts more water on top of the salty solution than the pulling power can hold and thus the water flows outwards into the interstitial space.
Same thing as "salt" solution happens with proteins like albumin in blood plasma. The venous side is the "salty" analogous side because it has all the albumin. The capillary wall is the semipermeable membrane that prevents albumin from crossing. The interstitial space is the watery side of the U-tube analogy. There is a constant battle between hydrostatic and oncotic to push blood and pull blood back into the venous side of the capillary "membrane" respectively.
In conclusion:
Oncotic pressure ~ osmotic pressure
Hydrostatic pressure = blood pressure from heart
Oncotic =/= hydrostatic or Blood pressure
perhaps you're mistaken. Remember how hydrostatic = oncotic there is no net fluid flow? this suggests that they are both forms of osmotic pressure.
for contrast:
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water.
given this explicit definition, what do you think?
