Hydrostatic pressure is caused by the flow of blood through vessels; it is the force exerted from within on the vessel walls. Osmotic pressure is the pressure exerted on the vessel from the OUTSIDE by the interstitial fluid. At the arteriole (closer to the heart) end of a vessel, hydrostatic pressure is greater than osmotic pressure, so fluid from the vessel leaks out. Thus, as you approach the venule end of that vessel, since fluid leaked out of the vessel before (and because you are farther away from the heart), the osmotic pressure is now greater than the hydrostatic pressure, so most of the fluid returns back to the vessel.
Regarding your question about osmotic P increasing and hydrostatic P unchanged, I don't even think that is possible given the nature of vessels. But if it were possible, with everything else held constant, blood volume would increase.
