You will need to cover college fees, accommodation and normal living expenses. For the 2017-18 academic year, the range of likely living costs is between £1,002 and £1,471 for each month spent in Oxford.
No, but you should have an idea about possible sources of funding that you might apply to. If you apply by the January deadline, you will automatically be considered for the Medical Sciences Division funding competition, but you should note that this is very competitive. Your place will be conditional on being able to provide evidence that you can cover fees and living costs for your first year, and having funding for any subsequent years.
You need to show that you have funding to cover you for the duration of the course. If you have only two years of funding, one option is to start on an MSc by Research, and transfer to a DPhil if you find further funding at a later date.