In order to add Related List to the Opportunity Page Layout, follow below steps. · One of the requirements is that the respective Custom Object should have a relationship (Master Detail or Lookup) with the standard Opportunity object. You can accomplish this by creating a custom field of type Master Detail or Lookup on your Custom Object where Opportunity is the parent . · Next step would be navigate to Setup > Opportunity in Quick Find within Side Panel > Page Layouts > Edit > Related Lists from the top panel > Drag and drop the appropriate related list from the top panel to the page layout > Save.