Bonsai trees from Oak may be tricky. Oak have a deep tap root, and will therefor need a serious root-trimming when collected from the wild. But even when one sows and acorn, the taproot may grow quicker than the above-ground sapling. So preparation is required when sowing oaks.
What is do is sow the acorn on top of a tile, burried in the ground; just like you would do when planting saplings in full ground to grow a trunk. On top of the tile I place a thin layer fo soil (Maybe 1cm) and place the acorn under a topdressing of 5cm. When the seedling germinates, the root will encounter the tile, and grow sideways. This will create a nice flat root system.
As soon as the sapling is >10cm, I place the first wire on the plant and give the trunk a twist for an interesting shape when the plant grows up. As soon as the shape has set (Typically after 1-3 months) I remove the wire, and take the sapling out of the ground. At this point I already clip of the growing tip of the taproot, to stimulate side-roots. I then place the sapling back in the ground, quite deep, with the roots on top of a impenetrable layer, such as plastic. This will keep the roots growing flat.
Images of a sapling I just dug up today.:
