Yes it is compulsory to have a mentor under the obu rules. The mentor is basically required to carry out a specific role details of which are available in the RAP preparation guide available on the ACCA website.
You needs to have a mentor because he will guide you and direct you towards your RAP. His responsibilities are to:
1. Feeback on your work 2. Review your work 3. Guidance about RAP preparation 4. Help in any issues you may face 5. Polish your interpersonal skills 6. Sign off your project certifying that it is your work.