They attempt to design the admissions process so tutoring doesn’t give you an advantage since not everyone can access it. The success of this is debatable but it is perfectly possible to get into Oxford without tutoring. The information a tutor can give you is probably in most cases available for free online if you do some digging. I also think you have been somewhat misled about the difficulty of the admissions process - they absolutely do not expect you to know things you have not already covered, and indeed may ask what you have done pre-interview to ensure your interview is not going to include content you know nothing about (unless the intention is to see how you approach new material). Oxford admissions are difficult, sure, but they’re difficult in such a way that there’s only so much a tutor can actually do for you. Having a tutor absolutely will not guarantee you get in, so I guess think about the expense and how you would feel if you didn’t get in after all that. If you’re at a state school you may be able to get free mentoring from Zero Gravity which will help you prepare rather than coach you, and if you are worried as a state educated student your school might not be much help I’d really recommend it. If you’re at a private school I’ll wager it’s quite likely your school will provide you with help preparing. Either way, speak to your school about what support they typically offer and do some research for free Oxford admissions resources before deciding on whether to get a tutor. Oxford admissions are to be honest quite subjective, there’s no one way to get it right and one tutor’s opinion on how to get ahead is probably very different to the next.
With admissions tests, there’s plenty of guides and past papers available online for free. The interview itself is a personal thing, and i don’t think it’s necessarily the best idea to have someone telling you what to say/do. Practicing a mock interview is useful, but you can find questions online and to practice talking through it anyone could run a mock interview for you - a teacher perhaps.
IMO the only part potentially worth getting a tutor for is entrance exams if you’re really struggling/there’s lots of unfamiliar content and you need someone to walk you through it but even then you could probably self teach and there are likely gonna be preparation books out there that will cost a fraction of what a tutor does. So really it’s up to you, but you do not need a tutor to get in and most people at Oxford that I know including myself had no tutoring and got in just fine!