What is the molecular mechanism behind oncotic pressure?
The molecular basis of osmotic pressure (oncotic pressure is a special type of osmotic pressure) is similar to filtration but on a very small scale.

Some membranes are partially permeable: they let small molecules pass but block larger ones. This asymmetry means that a flow of small molecules is created. Imagine things at the molecular level where a lot of individual motions are essentially random. The big molecules can't pass through the membrane but small molecules like water can pass through. If one side of the membrane consists of small molecules only and the other side has small and large molecules, then random motion will tend to lead to a net flow from of small molecules into the side containing large ones just because the concentration of small molecules is lower on the large molecule side. There are fewer opportunities for random small molecules to move from that side to the small molecule only side because there are fewer small molecules per unit volume on that side.

Obviously I'm simplifying a little, but this is the essential reason behind osmotic pressure.