These are interesting questions which do not have  simple and short answers. 

The IUPAC recommended definition of overpotential, η

"Deviation of the potential of an electrode from its equilibrium value required to cause a given current to flow through the electrode"

The term "electrode overpotential" is well described in https://en.wikipedia.org/wiki/Overpotential#Reaction_overpotential

You need to explain what is an "oxidizing power"

Water splitting  to ozone and H2 is highly thermodynamically unfavorable and requires  2.07 V. Water splitting to O2 and H2  has a much lower potential, 1.23 V.  

When you ask 

"Can electrodes with overpotential of 2.1V form ozone from electrolysis of water?"

you probably mean a potential versus NHE applied to anode. Theoretically this is possible, practically O2 will be the dominant product. 

These are interesting questions which do not have  simple and short answers. 

The IUPAC recommended definition of overpotential, η

"Deviation of the potential of an electrode from its equilibrium value required to cause a given current to flow through the electrode"

The term "electrode overpotential" is well described in https://en.wikipedia.org/wiki/Overpotential#Reaction_overpotential

You need to explain what is an "oxidizing power"

Water splitting  to ozone and H2 is highly thermodynamically unfavorable and requires  2.07 V. Water splitting to O2 and H2  has a much lower potential, 1.23 V.  

When you ask 

"Can electrodes with overpotential of 2.1V form ozone from electrolysis of water?"

you probably mean a potential versus NHE applied to anode. Theoretically this is possible, practically O2 will be the dominant product. 

Hi Orren, 

I think that you talk about the oxygen overpotential of the electrode used as anode. As Yurii has perfectly explained, the overpotential is the deviation from its equilibrium value. In the case of oxygen overpotential, it means the deviation from the 1.23V (vs NHE) from which the oxygen evolution reaction appears. 

In your case, your electrode has an overpotential of 2.1V (vs NHE). This means that the oxygen needs to add 0.88V to its equilibrium value to begin to produce oxygen on the electrode surface. Generally, in electrooxidation anodes with high oxygen overpotential are used in order to increase the process efficiency. 

However, the electrodes does not have "oxidizing power", they may act on pollutants degradation by two principal phenomenons:

In function of the kind of anode you use and your operating conditions, one of this processes will be your dominant oxidation step.

If you need more information I suggest you to read some bibliography.