You already know it's alive by having cut it (! - a little scratch would have told you the same thing) and found it green, but the mistake is putting it indoors where it doesn't belong and won't live. You say it doesn't show any signs of life, but it's dormant now (or at least it was until you brought it indoors) and when dormant, shows nothing til spring.

Thanks!
But i thought for plants spring is when ground temperature goes over 10-15 degrees. And my previous species showed its living by opening a buds in several days when i brought it from outdoors!

Spring is different everywhere in the world in terms of 'when', and depends on a lot more than ground temperature - things like sun, hours of sun, rain, frost, air temp., etc. all matter. Plants also respond to 'spring' at different times - some buds open early (early March), others not til June. If you've brought other trees inside in the past and the buds opened, well, that's them, but it would have depended on what they were, when you did it, where you live (where do you live?), etc., but it's still January, and just not even close to the time where you should have brought anything indoors. And in any case, trees like oaks, maples, conifers/evergreens, anything that normally lives outdoors all year cannot survive indoors no matter what. You may keep a few alive for a short time, but that's not really the point is it? It's not just about dormancy, or temperature, or any one thing, but a lot of factors that only nature can organize just right for them to keep living a long time.

Lucy is right, the best thing to do is put it back outside and wait until Spring to see how it does.

How big is the pot though? If it's on a windowsill I assume it's pretty small so you might want to wrap it up in something when it's outside (or else leave it in the garage/garden shed) until it's warmed up a bit - It's not very good if the soil freezes solid.

Thanks for answers!

I live in Kyiv, Ukraine. And it's about from -5 to 0 degrees outdoors, at the balcony I have from 8 to 10 degrees. And in the room it's 25 degrees.
I agree the pot is rather tiny, but it suits well for tree's rootball so that water might not retain too much (well, if it is absorbed by roots). The diameter of upper edge of the pot is 19 cm, the diameter of base is 11 cm, heights is 13 cm. Anyway, if tree survives, I'll repot it in several years.
The window-sill is lit with daylight lamp 11W. It gives more light than the sun for the window-sill, because it's cloudy nowadays.
There was no point in brining tree in the middle of winter (i'm going to gather some new trees in spring, when buds are about to open, would it be right?), but i wanted to try, what conditions tree could survive, so that i would know in future how to handle it with no fear of killing it by accident. It is not so sad if tree dies at the beginning of the training than if it dies after several years of training because of wrong timing of repotting.
I believe indoor tree can live long and happy time with adequate treatment. Why do you think indoor trees can't live long periods of time? at the house i can control every factor: lighting, soil, temperature, watering, size of pot, fertilizers, airing etc. I just want to learn how to diagnose tree's state correctly and reply to their needs in time.

Max, if 'outdoor' trees could live indoors, then bonsai as a 500 year old art (and science) would be very, very different today. There ARE trees that can live inside, but they are definitely tropical ones, and you need to provide extra lighting and high humidity, etc. It has been proven over and over and over again that trees that go dormant (below freezing) for months in winter canNOT live inside beyond a few weeks or so, and you would be wasting your time and the tree's life to keep going against nature to try it. You CANNOT control the things they need - artificial light, inadequate temperature, lack of serious wind (to help strengthen trunks and blow away pests and old foliage) cannot be controlled. Fertilizers don't make up for all the tiny amounts of minerals and things nature provides outdoors, and neither can a little fan or misting. You better keep your tree INside now as it's too late to go cold again (2 days is as long as it takes to warm up), but as soon as there is NO danger of frost anymore (late April), put it back outside and LEAVE it there for life!

You can control all the factors in side your house, but in order to create best environment for your tree you need to open all the windows and turn the heating off !!! - most people don't like to do this, so they put the tree outside!

Your oak tree might survive inside for a few years, but if it's too warm for it to go dormant it will eventually get 'tired' and die out :(

I think you can keep it on the balcony - it gets fresh air but you can still go and see it easily.

ps. I do not think the pot is too small for the tree, only that if you put it on the ground outside in the Ukraine winter the soil would probably freeze, so you need to be careful.

I don't know, but don't you want to say that every serious bonsai is developed open air and there is NONE that lives only indoors (except for some tropical ones)?
Okay, I can't provide exactly the same conditions for the tree indoors that it usually has in nature. And as you speak, one of the greatest problems is dormancy for deciduous species. But if i provide dormancy by leaving it on -5, 0 degrees temperatures during winter and preventing soil from freezing, will it make the Oak live longer?

Besides, i have a six-year-old ficus tree which has lived its sad life in desperate conditions. Despite of this it is about one meter high! I understand that ficus is a tropical tree and is often used as decorative plant at homes, but my ficus was in conditions not even close to natural ones. And there are a lot of examples when some species survive in conditions they weren't meant to. For instance, a pine survived in rocky mountains having its roots in some moss and fallen needles. I believe accommodation is possible...

Max, Max, Max... 'some moss and fallen needles' (in rocky mtns) is prime stuff for a pine IN NATURE! Try to duplicate that atmosphere at home and you'll quickly have a dead tree. Ficus are tough, and if conditions weren't ideal, they were still obviously in the ballpark - e.g. INdoor conditions, lack of fussing and/or overwatering, I'll assume, who knows (you haven't actually said), but it's not the same at all. And just keeping your oak's soil (whatever mix it might be - and that matters tremendously) at a certain temperature won't do it. Will not. And keeping the soil from freezing is not the point! Soil freezes hard all the time out there, and everything lives quite happily. Unless you're some divine spirit who performs miracles that horticultural experts, never mind internationally renowned bonsai ones, have yet to conquer, I suggest you stick to Ficus (or African violets, or philodendron).

Ok, I give up! The Oak's small branches are drying out.I don't think it will survive by spring anyway. I decided to move it to balcony, as hannahbelle suggested, at least it's not so dry out there, though temperature's a bit low... Practically, Oak's dead.

Violets or Philodendron??? I wanted to have a tree with thick trunk! Ficus is a better solution, but i read it reacts badly on pruning. Widely suggested ficus is Chinese Banyan, but where can i get this? However, i will try to find it.

Are there any tropical trees, not flowers, suitable for home conditions?

Hi, there are quite a few actually, but I don't know what's available to you there. I think part of your problem (Ficus reacts badly to pruning? No, it doesn't, not if you know what you're doing.) is that you a) have been given bad information to begin with, and b) you just need to learn more about growing things, indoors and out. So I really think you need to start reading - and www.bonsaihunk.us/cultural.html is a great place to start, for tropicals (and ficus), and www.bonsai4me.com is another - less depth, but a wider range of trees. Also try www.evergreengardenworks.com - very good basics on (mostly) outdoor trees, particularly evergreens. I was being a little sarcastic about the houseplants... sorry!

Another tree used for Bonsai which can be grown indoors is Chinese Elm (latin name: Ulmus Parvifolia). Also, there are a _lot_ of different types of ficus, check out the resources Lucy suggested.

Good luck;- hope the oak recovers.