..."above a certain level of applied potential, system hits the mass transport limit. Then, bulk concentration of O won't change the current density. "...I don't agree...

I will say that the current density is proportional to the reactant bulk concentration if (and only if) the system hits the mass transport limit (i.e., constant diffusion rate at electrode surface).

Thank you Cornel for your comment. At anodic potentials (positive), when system hits the limit, would the current be proportional to the concentration of the product of oxidation? Please correct me if I'm wrong but I think after the limit, current would be proportional to bulk concentration of R (reactant in the anodic direction) not O (product in the anodic direction).

I think that both cases-limitations maybe valid[1] depending on the relative processing speeds and the concentration.

1. Only one of them is the master limitation in each case.

I agree with this: "after the limit, current would be proportional to bulk concentration of R (reactant in the anodic direction) not O (product in the anodic direction).".

If the applied potential is high enough, then R->O+ne is favoured (high rate) and you may neglect O+ne=R and its rate. The overall rate will depend only on R bulk concentration.

If you use a cyclic voltammetry, in a cyclic voltammetry experiment, the solution is kept unstirred; in this situation, mass transport can occur only by diffusion due to concentration gradients created around the electrode surface.

If it is a reversible system,

I will use this eq.

Randles–Sevcik equation:

Basically reversible (diffusion-controlled) and irreversible (charge-transfer controlled) kinetics of electrode process:

Peak Current (i) will be proportional to square root of scan-rate.

i = v1/2  , simplified form of RS eq.

Hope it helps!