Yes, candidates who have obtained UG degree from an Open University can appear for the Common Admission Test (CAT), and can get admission to IIMs as well. According to the official website of the CAT, the minimum eligibility criteria to appear for the entrance exam is that candidates must hold a Bachelor’s Degree with at least 50% marks or equivalent CGPA (45% marks for candidates who belong to SC, ST, and PwD category) awarded by any of the university incorporated by an act of the central or state legislature in India or other educational institutions established by an act of Parliament or declared to be deemed as the university under the section 3 of the UGC Act, 1956, or possess an equivalent qualification recognized by the Ministry of HRD, Government of India.