What's the difference between wave propagation velocity and particle oscillation velocity?

It might be easier to start with the difference in the context of a surface gravity wave in water.

When a water wave passes a point, to first order, the water particles move in closed vertical circular paths whose radius decreases linearly with depth (circles get smaller as you increase depth.)

The water wave moves at a constant velocity perpendicular to the depth whereas the water particles merely oscillates in place.  So while the two speeds are related, the two speeds are generally different.

The same concept applies to electromagnetic waves that encounter a medium.  They cause charged particles to oscillate with zero net displacement as the wave passes.