The eligibility criteria for OBC candidates in the UPSC Civil Services Examination include being a citizen of India, meeting the age limit set forth by the commission, and possessing the required educational qualifications. OBC candidates must also provide a valid caste certificate issued by the competent authority to avail the benefits of reservation. The commission has designated a specific age relaxations for OBC candidates, which allows them more attempts compared to the general category, facilitating increased chances for success.