It may not be ideal, but as long as you get active fermentation, the CO2 offgassing will displace the O2, just may take longer. I just used my 30L speidel for the first time, and it took an extra 24 hours to see airlock activity, though I could see the wort level rising in the fermenter( krausen that is not wort). Its just a lrger amount of O2 to displace than in a more normal scenario