TL;DR: Do not complain outright to your examiners about the supervision. If they are any good, they will work it out for themselves.
The purpose of a viva is several fold:
- Make sure the student did the work they have written about in the thesis.
- Make sure the student understands the work they did.
These first two control for situations where the supervisor basically did the PhD for the candidate, and just used them as hired hands to do technical or leg work.
- Probe the students broader understanding of the field and how their work relates to the field
- Make sure the student understands, or can be made to understand the limitations and the shortcomings of their work.
- Check that the written thesis is a fair representation of the abilities of the student.
Often examiners come into an exam unconvinced by the thesis, but the exam convinces them that the student is capable, but that they just didn't write the thesis very well.
If you feel the thesis work is not good (whether that is the fault of the supervisor or not), the viva is your time to shine. So, make sure you are confident of exactly what the strong and weak points of your thesis work are. You say you doubt its originality. Don't doubt, know. If someone else has done this before, know who, and exactly what they did and exactly where your work is similar and where different. Don't think you work lacks depth and analysis; know which ideas are not fully developed, and the directions in which they could be taken if you had more time. If you abandoned ideas uncompleted, be prepared to discuss the pros and cons of this, and say if on balance you made a mistake in abandoning or not.
Most of all, be prepared to say where you were wrong in the past. Not a general, handwavy kind of "oh I wasn't very good then", but specifics: I thought this, but actually that was the case. At the time I felt topic X was not going anywhere, but I've since come to learn that A, B and C could have allowed me to progress it.
It is highly unlikely you will outright fail. But you may well be asked to make quite substantial corrections if the thesis really is as bad as you say (students, and some supervisors often have an inflated view of what is necessary in a thesis). Think ahead what these might be, and have some ideas for them already worked out.
All this demonstrates your quality as a scholar or scientist, independently of what your relationship with your supervisor has been. Which, in the end, is what is being tested in a viva.
Traditionally it is the job of the external examiner to really probe the work, and the job of the internal examiner to a) act as umpire between the external and the candidate b) be aware of the context in which the work was done. You might be surprised about the extent to which the internal is aware of problems with supervision (especially if they are from the same department). But also, good examiners will get a feel for the supervision in the exam, in science vivas especially, the exam is as much an examination of the supervisor as it is the student. You won't pass with substandard work because your supervision was poor, but you might be cut some slack in making it better.