Just simply add '친'(chin) before '오빠'(oppa). which is, '친오빠' you can apply this to any terms that refer to your family members. For instance, '친형' (= older brother in your family called by male) '친누나'(= older sister in your family called by male ) '친언니'(= older sister in your family called by female) '친동생'(= younger brother or sister in your family, in this case it is irrelevant to the gender of the caller) etc.

Just add '친' in front of 'oppa'. ex) 친형, 친오빠, 친누나, 친동생