South Swedish beginner here!

So i planted this acorn in soil last year and kept it indoors at first. Then when late spring/early summer came I thought I would put it outside. Basically we had some cold nights and the lil tree started looking beat up, browning leaves and some falling off.

So I took it inside again thinking I killed it, and forgot about it for awhile. Watered it once in awhile keeping hopes up for a few weeks, then forgot it completely when no signs of life were there.

Yesterday evening i noticed the lil guy had managed to pop up 3 new leaves after this dormancy period, has to be right? And now I don't know what to do. Fall is around the corner and this being an oak should be kept outside, should i start getting it used to being outdoors for the colder months or will that kill it at this state? What should I do? Looks like he thinks he is in spring times right now.
Thanks in advance!

I don't think you put it into hibernation, you had it inside. That was the problm. It is still summer so it should be outside never inside.

I would place it outside. Main thing is to prtect it from direct sun and wind for the first few weeks though.