You haven't mentioned which course you are enquiring about. Here are the eligibility criteria for OJEE B.tech 2019: 

    • Aspirants must have JEE Main score. 

    • Passed 10+2 with Physics and Mathematics as compulsory subjects and any one of Chemistry / Biology / Biotechnology / Technical Vocational Subject / Computer science / Electronics / Geology / Information Technology / Statistics.

    • The minimum percentage in class 12 for B.Tech courses is 45 % and 40 % for SC/ST candidates. 

Now, Diploma students also can appear for JEE Mains. And ideally Diploma students can get a lateral entry in B.Tech. So you should be eligible for B.Tech with your Diploma Certificate.