I always assumed surface oxygen was given to prevent further damage from DCS and it wasn’t until recompression on O2 could DCS be treated
100% O2 has a number of benefits.
1. It improves the pressure gradient, between the Nitrogen in the tissues, and the Nitrogen gradient in the surrounding tissues and blood, encouraging Nitrogen to diffuse faster into the surrounding tissues and blood and be expired.
So it helps you get rid of the Nitrogen quicker and reduce the bubble size quicker. Potentially returning circulation to areas impeded by the bubble(s)
2. It saturates the blood and surrounding tissues with 100% oxygen. This allows oxygen to perfuse past the restriction and oxygenate (some) of the surrounding tissue. Reducing tissue oxygen starvation down stream of the restriction.
3. It compensates for limited efficiency of the lungs and circulation system, where the lungs or circulation system have been damaged.
I am sure someone else will add to this.
NOTE
There is often an apparent worsening of the situation when a diver suffering from DCI is put on 100% O2. Ignore this.
Gas them Hard and gas them quick.
Do not ration O2.
Never forget Administering O2 is ONLY A FIRST AID TREATMENT, you still need to evacuate them to a hyperbaric facility and medical care.
Remember if you are out of O2 use the next richest Nitrox.