onset of oxidation for cyclic voltammetry
Please don't apply the reversible CV diagram on something which is clearly a non-classical cyclic voltammogram. When you experimentally don't get the shape as predicted by the theory, it means it that particular theoretical model does not apply in your case.
Look at three different types of voltammograms in this picture. Does your CV look like a reversible one? If not, then you were comparing an apple with an orange.

There is a very nice article which is relevant to you case:
Practical Aspects of Cyclic Voltammetry: How to Estimate Reduction Potentials When Irreversibility Prevails,  Journal of The Electrochemical Society, 2019,  166(5):H3175-H3187
This figure is relevant:

Examples of cyclic voltammograms showing chemically reversible and irreversible oxidation with designated potentials: E a = anodic potential, E c = cathodic potential, E (i) = inflection-point potential, E (p/2) = half-peak potential, E (e) = edge potential.