I'm brewing my second batch and had a bit of an accident yesterday. I brewed 20l two days ago and when I checked my fermenter (bucket) yesterday it had overflown. I don't know what possessed me to open the lid and clean in, instead of just cleaning the mess and airlock, but I did. The bucket was open for maybe 10-15mins. I did clean and sanitize both lid and airlock, so hopefully there's no contamination, but I am worried about oxidation. Should I be? When and how will I know if it happened? Is there any way to fix it?
It's fine. I assume the beer is still actively fermenting, in which case not only will freshly-produced CO₂ (somewhat) displace the O₂ in the headspace, but the yeast can still clean up any O₂ that does dissolve into the young beer.
Many high-gravity beers actually forcibly inject O₂ during the early stages of fermentation to get a solid ferment.