AndreChe,
Do you know that dry flow or the wet flow rate as a total? If it's dry flow and you also know volume of water vapour then it's easy just recalc the % by determining dry volume flow and dividing by new humid flow rate.
If it's a wet flow then you need to calculate the volumetric ratio for your stream which can be done from knowing pressure, T and humidity ratio, or if you know water vapour volume flow. You can directly correlate the dry flow rate from this data and using this recalc your %O2 as a wet basis.
I don't know any other way that's easier I'm afraid. I have a thermal balance calculation spreadsheet that I use for enthalpy calculations in my absorption column calculations if you have traouble I can copy the relevant section of the calcs and send through to you by email, let me know how you go.
Cheers,
K.