Bonsai projects are great! I have many running at the moment too. Also some oak, but mine are branching already... I see a few problems here. First of all, this tree (plant) needs to grow outdoors, not indoors. It needs sun, rain, and wind, but most important cold winters as well. It needs to lose its leaves in winter. Branching occurs very often in spring right after winter. So my advice is to give it its natural environment back, put and keep it outdoors. But let it slowly adjust (acclimate) to full sunlight first, by giving it a little bit more sunlight each day.
Second advice, is to let the tree first grow bigger and bigger (without the root trimming). It takes about 7 years before the stem is thick enough for bonsai. Then after these 7 years, you shape it and put it in a real bonsai container.
Good luck with your project!