A great article from 2005 summarises this nicely. There are five major mechanisms: bulk convection, Pendelluft mixing, Taylor dispersion, coaxial flow and augmented molecular diffusion. Question 23 from the second paper of 2010 and Question 15 from the first paper of 2002 have asked about HFOV during the golden age of oscillation, in the pre-OSCAR and OSCILLATE era.
There are five major mechanisms:
The whole of normal human ventilation can be described as "bulk convection". The definition of bulk convection is the movement of a group of molecules though a medium of other molecules. In short, this is tidal volume, the actual entry and exit of gas through the patient. In HFOV, where the "tidal volume" is negligible, bulk convection is represented by the continual entrainment of fresh gas, as oxygen is absorbed at the alveolus. The decreased pressure generated by continual gas removal makes space for more gas.
This strange German word describes the movement of gas from some alveoli into other alveoli. Basically, it is the exchange of gas between lung units. This happens when some neighbouring regions have very different compliance; their time constants will differ and they will "blow" gas into each other when one is collapsing and the other still remains open. Some engineers were able to demonstrate the mechanics of this in a model of airway bifurcation with adjustable "bronchial" diameter.
This is a complicated concept, which essentially describes the increase in the diffusivity of a substance which is subjected to shear forces. The forward push of the oscillator results in a jet of fresh oxygenated gas which is pushed down the centre of the airway; this jet then diffuses into the gas at the periphery of the bronchi, enriching it with oxygen.
Gas on the outside of a tube flows more slowly than the gas positioned centrally inside the tube. Because of this, in HFOV there may actually be a bi-directional flow of gas, with the gas on the outside moving in the expiratory direction even while the central gas moves in the inspiratory direction.
The violence of the oscillator imparts such force to the gas, that molecules find themselves pushed to and fro with substantially greater force. The mechanical energy of the oscillator, thus imparted to the molecules, results in an increase of their velocity, and this enhances diffusion of gas though a gas-filled compartment. In short, it is a more vigorous Brownian motion.