not custom fields on standard objects like opportunities, not at this time.
you can upload / download custom links on the Opportunity object, then the Admin can download and place those on the pages that need it, therefore the Admin has the final step to make this link visible.
you can add objects which relate to the opportunity
you can upload / download custom links on the Opportunity object, then the Admin can download and place those on the pages that need it, therefore the Admin has the final step to make this link visible.
you can add objects which relate to the opportunity