Opportunity have a related list some x object , and having buttons a,b.these buttons are visible in classic mode. But in lightning the related list object x showing edit and delete buttons or links , if I open the record of that related list those a,b buttons are visible . How to show the a,b buttons in the place of edit and delete links
Adding a screen shot or more detail to your question would be a good idea.
Unless I misunderstood your question, AFAIK, you won't be able to add quick actions to lightning related lists. See this idea.