I am sorry jan pop, The terminology of onset potential in gas evolution reaction is not valid on. Because it doesn't give meaningful result. For that, you depend on the potential, which gives 10 mA.cm-2 current. Please keep it onset potential of OER, HER, chlorine / bromine evolution reaction is not good. We couldn't get exact onset value from Linear polarization curve (That is the draw back). 

For potential correction 

Your reference electrode potential (Ag/AgCl) = 0.194V

1M KOH pH roughly around 14

Hence 14*0.059=0.826

0.826+0.194= 1.02 Vs RHE

Hence you have got 10 ma.cm-2 at the potential of 0.500V vs Ag/AgCl. It is equivalent to 1.502V vs RHE. 

If you know the pH of your solution you can calculate the thermodynamic potential for oxygen evolution using Nernst equation. The standard potentials for the respective electrode reactions you'll find  and the electrochemical series. However, depending on the electrode material a certain overpotential for the real onset must be considered. Therefore the onset potential is more oer less shifted to more anodic potentials.

Assuming a 1 molar KOH solution the redox potential for O2 evolution is +0.401 V vs. SHE, the potential of your reference Ag/AgCl is +0.207 V vs SHE (+- a few mV depending on filling electrolyte). This yields a thermodynamic potential of 0.194 V for O2 evolution versus Ag/AgCl. The real onset potential will be somewhat higher due to the overpotential. Asuming a smooth Pt surface and a current density of 10 µA/cm^-2 the overpotential amounts to about 0.5 V yielding in a potential of approximately 0.7 V vs Ag/AgCl in total for this current density. 

Onset potential (for a reaction) is an ill defined potential; any potential value exceeding the Nernst potential can be justified to be one. 

Such usage is to be discouraged in scientific discussions. Any such values, serve if at all, to give instructions to unskilled labor on shop floor.

I am sorry jan pop, The terminology of onset potential in gas evolution reaction is not valid on. Because it doesn't give meaningful result. For that, you depend on the potential, which gives 10 mA.cm-2 current. Please keep it onset potential of OER, HER, chlorine / bromine evolution reaction is not good. We couldn't get exact onset value from Linear polarization curve (That is the draw back). 

For potential correction 

Your reference electrode potential (Ag/AgCl) = 0.194V

1M KOH pH roughly around 14

Hence 14*0.059=0.826

0.826+0.194= 1.02 Vs RHE

Hence you have got 10 ma.cm-2 at the potential of 0.500V vs Ag/AgCl. It is equivalent to 1.502V vs RHE.