Anywhere from bud swell (earlier in warmer areas, I start in mid winter here with only light frost) right through to small green shoots is OK.
Older oaks do not seem to like having lots of roots removed. Seems to be better to repot lightly more often than a hard prune less often. Seedlings do not worry bout root removal. I have cut the main root completely off a germinating oak and new roots grow from the root stump. Now I usually chop the new root about 2-3 cm long to get better radial roots on germinating Q. robur.