Wish I could help, but can only admire the growing of Bonsai trees. Sorry if some one has not taken your question seriously.

Mic I've already responded you can't keep the top half of a tree and remove the trunk below it. Trees grow from their roots. I am not sure if you can bonsai an oak but you would need to do so when it was small with root pruning and restricting the pot size.

A plants grow from the bottom up, so if you want a bonsai oak you need to keep it controlled form the get go.

Hi Mic1664 I didnt see your question when you first posted it. I think what you are trying to do isnt possible. firstly bonsai trees are grown from seed and then pruned as the plant grows so a mature bonsai oak will be hundreds of years old. If youd mentioned your love of bonsai that would have clarified your question.
if you look there are often questins that we answer with more questions.

How old/large is your oak? to cut out a section of a trunk would kill the tree's top growth. even if you could rejoin the top and lower sections the water carrying vessels will not marry up. I am a biologist of 30yrs + but I still ask for help in areas where I have little experience. So please dont feel 'less clever'.

So can I also say non of us are perfect but many of the members here have good friendships and often they spring banter off each other as a result of long standing humour. I dont think it should be taken seriously.

Mick - please appreciate that this (and most other) forum suffer a fair few wind up questions, and sadly owing to the phrasing of your question and there being no mention of bonsai the wrong impression was formed.

So to be clear, you would like to know if it's possible to take a cutting from your oak to start a bonsai project?

In short, yes. But my friend says that it needs to be done in the autumn from semi hardwood cuttings and it takes a very long time. Far better to start with an acorn he said..........