Carbonation level changes alone will not cause oxidation. You need to look at all of your beer handling procedures with an eye towards "can any O2 get in the beer or headspace with this operation?" Getting O2 in the beer is the worst, as you can't get it out again, and it will eventually oxidize the beer. If you get O2 (air) in the headspace, then you need to pressurize and vent a sufficient number of times to reduce the O2 to insignificant levels. To be safe, assume if the keg is open, the headspace becomes all air in just a few minutes. There is no protective blanket of CO2 (it's a myth propagated by those who don't understand gas diffusion.) The table and chart below show the percent of remaining O2 in the headspace after different numbers of purges at different pressures. Air is 21% O2 (0.21 fraction) and you can use this number to calculate ppm of O2 remaining after purging. I'll do and example below.
If you do 8 purge cycles at 15 psi, you will have 0.36% of the original O2, or 0.0036 of the original. 0.21 * 0.0036 = 0.000756 or 756 ppm O2 in the headspace. This is probably ok for storage of a few months. For long term storage, you probably want to get the O2 down to 10 ppm (9 purge cycles at 30 psi.)
If you don't like doing purge cycles, you can fill the keg
completely with StarSan, and then push the StarSan out with CO2. Then fill the keg thru the liquid out post (open only the pressure relief valve to allow filling.)
Brew on
